selftests/hid: force using our compiled libbpf headers

Turns out that we were relying on the globally installed headers, not
the ones we freshly compiled.
Add a manual include in CFLAGS to sort this out.

Tested-by: Nick Desaulniers <ndesaulniers@google.com> # Build
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230825-wip-selftests-v3-3-639963c54109@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

tools/testing/selftests/hid/Makefile

@@ -22,6 +22,8 @@ CXX ?= $(CROSS_COMPILE)g++
 HOSTPKG_CONFIG := pkg-config
 
 CFLAGS += -g -O0 -rdynamic -Wall -Werror -I$(OUTPUT)
+CFLAGS += -I$(OUTPUT)/tools/include
+
 LDLIBS += -lelf -lz -lrt -lpthread
 
 # Silence some warnings when compiled with clang