tests: fix build on rpi with EGL

This patch fixes the compiler errors:

  CC     weston_test_la-weston-test.lo
weston-test.c:34:21: fatal error: EGL/egl.h: No such file or directory

  CC     buffer-count-test.o
buffer-count-test.c:30:21: fatal error: EGL/egl.h: No such file or directory

On rpi, the EGL headers are not in the standard path.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

tests/Makefile.am

@@ -69,6 +69,7 @@ weston_test_la_SOURCES =		\
 	wayland-test-server-protocol.h
 
 if ENABLE_EGL
+weston_test_la_CFLAGS += $(EGL_TESTS_CFLAGS)
 weston_test_la_LDFLAGS += $(EGL_TESTS_LIBS)
 endif
 
@@ -121,6 +122,7 @@ subsurface_weston_SOURCES = subsurface-test.c
 subsurface_weston_LDADD = libtest-client.la
 
 buffer_count_weston_SOURCES = buffer-count-test.c
+buffer_count_weston_CFLAGS = $(GCC_CFLAGS) $(EGL_TESTS_CFLAGS)
 buffer_count_weston_LDADD = libtest-client.la $(EGL_TESTS_LIBS)
 
 xwayland_weston_SOURCES = xwayland-test.c