[PATCH] Allow building hid.ko as an "external" module

[Cristian Klein]

For quickly testing USB HID quirks with a larger community, it is useful
to be able to build hid.ko as an external module, e.g., against the
source code of the running kernel.

Before this patch this failed as follows:
```
$ make -C /lib/modules/$(uname -r)/build M=$PWD/drivers/hid
make: Entering directory '/usr/src/linux-headers-5.3.0-51-generic'
  CC [M]  /home/cklein/linux/drivers/hid/i2c-hid/i2c-hid-core.o
  CC [M]  /home/cklein/linux/drivers/hid/i2c-hid/i2c-hid-dmi-quirks.o
  LD [M]  /home/cklein/linux/drivers/hid/i2c-hid/i2c-hid.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ishtp/init.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ishtp/hbm.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ishtp/client.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ishtp/bus.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ishtp/dma-if.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ishtp/client-buffers.o
  LD [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/intel-ishtp.o
  CC [M]  /home/cklein/linux/drivers/hid/intel-ish-hid/ipc/ipc.o
/home/cklein/linux/drivers/hid/intel-ish-hid/ipc/ipc.c:12:10: fatal error: client.h: No such file or directory
   12 | #include "client.h"
      |          ^~~~~~~~~~
compilation terminated.
make[2]: *** [scripts/Makefile.build:290: /home/cklein/linux/drivers/hid/intel-ish-hid/ipc/ipc.o] Error 1
make[1]: *** [scripts/Makefile.build:519: /home/cklein/linux/drivers/hid/intel-ish-hid] Error 2
make: *** [Makefile:1656: _module_/home/cklein/linux/drivers/hid] Error 2
make: Leaving directory '/usr/src/linux-headers-5.3.0-51-generic'
```

Signed-off-by: Cristian Klein <cristian.klein@xxxxxxxxxxxxx>
---
 drivers/hid/intel-ish-hid/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hid/intel-ish-hid/Makefile b/drivers/hid/intel-ish-hid/Makefile
index f0a82b1c7..db4974c43 100644
--- a/drivers/hid/intel-ish-hid/Makefile
+++ b/drivers/hid/intel-ish-hid/Makefile
@@ -23,4 +23,4 @@ intel-ishtp-hid-objs += ishtp-hid-client.o
 obj-$(CONFIG_INTEL_ISH_FIRMWARE_DOWNLOADER) += intel-ishtp-loader.o
 intel-ishtp-loader-objs += ishtp-fw-loader.o
 
-ccflags-y += -I $(srctree)/$(src)/ishtp
+ccflags-y += -I $(src)/ishtp
-- 
2.20.1


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