Fix the kernel module build (config.h dir now included).

If libcap_types.h is a public header, it probably shouldn't include
config.h, or if it does, we'll have to rename it to avoid conflicts, but
this solves the build for now.

src/kernel/Makefile.in

 
 top_srcdir = @top_srcdir@
 abs_top_srcdir = @abs_top_srcdir@
+abs_top_builddir = @abs_top_builddir@
 
 COMMON_SRCFILES = \
 	$(top_srcdir)/src/common/cap.c $(top_srcdir)/src/common/cptr_cache.c
@@ -16,7 +17,8 @@ cap_module-objs := cap.o cptr_cache.o
 KDIR ?= /lib/modules/$(shell uname -r)/build
 PWD := $(shell pwd)
 
-ccflags-y = -DMODULE -D__KERNEL__ -I$(abs_top_srcdir)/include
+ccflags-y = -DMODULE -D__KERNEL__ -I$(abs_top_srcdir)/include \
+	-I$(abs_top_builddir)/include
 
 $(CFILES:.c=.o): $(HFILES) Makefile