build: honour libinput header location

Add the respective CFLAGS to the build, otherwise it will error out as
seen below.

src/libinput-seat.c:30:22: fatal error: libinput.h: No such file or directory

v2: add the CFLAGS only as needed, suggested by Pekka

Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Jan Engelhardt <jengelh@inai.de>
[Emil Velikov: polish commit message, v2]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

Makefile.am

@@ -178,7 +178,7 @@ weston_LDFLAGS = -export-dynamic
 weston_CPPFLAGS = $(AM_CPPFLAGS) -DIN_WESTON 		\
 				 -DMODULEDIR='"$(moduledir)"' \
 				 -DXSERVER_PATH='"@XSERVER_PATH@"'
-weston_CFLAGS = $(AM_CFLAGS) $(COMPOSITOR_CFLAGS)
+weston_CFLAGS = $(AM_CFLAGS) $(COMPOSITOR_CFLAGS) $(LIBINPUT_BACKEND_CFLAGS)
 weston_LDADD = libshared.la libweston-@LIBWESTON_MAJOR@.la \
 	$(COMPOSITOR_LIBS) \
 	$(DL_LIBS) $(LIBINPUT_BACKEND_LIBS) \
@@ -346,6 +346,7 @@ x11_backend_la_SOURCES = 			\
 	shared/helpers.h
 endif
 
+INPUT_BACKEND_CFLAGS = $(LIBINPUT_BACKEND_CFLAGS)
 INPUT_BACKEND_LIBS = $(LIBINPUT_BACKEND_LIBS)
 INPUT_BACKEND_SOURCES =				\
 	libweston/libinput-seat.c		\
@@ -369,6 +370,7 @@ drm_backend_la_CFLAGS =				\
 	$(COMPOSITOR_CFLAGS)			\
 	$(EGL_CFLAGS)				\
 	$(DRM_COMPOSITOR_CFLAGS)		\
+	$(INPUT_BACKEND_CFLAGS)			\
 	$(AM_CFLAGS)
 drm_backend_la_SOURCES =			\
 	libweston/compositor-drm.c		\
@@ -443,6 +445,7 @@ fbdev_backend_la_CFLAGS =			\
 	$(EGL_CFLAGS)				\
 	$(FBDEV_COMPOSITOR_CFLAGS)		\
 	$(PIXMAN_CFLAGS)			\
+	$(INPUT_BACKEND_CFLAGS)			\
 	$(AM_CFLAGS)
 fbdev_backend_la_SOURCES =			\
 	libweston/compositor-fbdev.c		\