Makefile: Allow specifying kenrel header path

Helps with doing builds on systems where the kernel headers are not
installed.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/Makefile b/Makefile
index 9d1f5b9..733511c 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,10 @@
 CFLAGS	= $(shell pkg-config --cflags libsystemd) -Wall -O2
 LDLIBS	= $(shell pkg-config --libs libsystemd)
 
+ifdef KERNEL_HEADERS
+	CFLAGS += -I$(KERNEL_HEADERS)
+endif
+
 EXE = btbridged
 
 all: $(EXE)