selftests: connector: Fix Makefile to include KHDR_INCLUDES

The test compile fails with following errors. Fix the Makefile
CFLAGS to include KHDR_INCLUDES to pull in uapi defines.

gcc -Wall     proc_filter.c  -o ../tools/testing/selftests/connector/proc_filter
proc_filter.c: In function ‘send_message’:
proc_filter.c:22:33: error: invalid application of ‘sizeof’ to incomplete type ‘struct proc_input’
   22 |                          sizeof(struct proc_input))
      |                                 ^~~~~~
proc_filter.c:42:19: note: in expansion of macro ‘NL_MESSAGE_SIZE’
   42 |         char buff[NL_MESSAGE_SIZE];
      |                   ^~~~~~~~~~~~~~~
proc_filter.c:22:33: error: invalid application of ‘sizeof’ to incomplete type ‘struct proc_input’
   22 |                          sizeof(struct proc_input))
      |                                 ^~~~~~
proc_filter.c:48:34: note: in expansion of macro ‘NL_MESSAGE_SIZE’
   48 |                 hdr->nlmsg_len = NL_MESSAGE_SIZE;
      |                                  ^~~~~~~~~~~~~~~
`
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://lore.kernel.org/all/CA+G9fYt=6ysz636XcQ=-KJp7vJcMZ=NjbQBrn77v7vnTcfP2cA@mail.gmail.com/Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/d0055c8cdf18516db8ba9edec99cfc5c08f32a7c.1690564372.git.skhan@linuxfoundation.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/connector/Makefile

 # SPDX-License-Identifier: GPL-2.0
-CFLAGS += -Wall
+CFLAGS += -Wall $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS = proc_filter