libweston: fix pixel-format helpers compilation on non-X11 EGL platforms

Since building libweston includes EGL/egl.h from pixel-formats.c,
EGL_CFLAGS must be added to libweston_CFLAGS, as on some platforms that
contains -DMESA_EGL_NO_X11_HEADERS, and fails to compile without it:

  CC       libweston/libweston_3_la-pixel-formats.lo
In file included from [...]/usr/include/EGL/egl.h:39:0,
                 from libweston/pixel-formats.c:39:
[...]/usr/include/EGL/eglplatform.h:119:22:
fatal error: X11/Xlib.h: No such file or directory

Fixes: 903721a621 ("libweston: Add pixel-format helpers")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Daniel Stone <daniels@collabora.com>

Makefile.am

@@ -72,7 +72,7 @@ install-libweston_moduleLTLIBRARIES install-moduleLTLIBRARIES: install-libLTLIBR
 lib_LTLIBRARIES = libweston-@LIBWESTON_MAJOR@.la
 libweston_@LIBWESTON_MAJOR@_la_CPPFLAGS = $(AM_CPPFLAGS) -DIN_WESTON
 libweston_@LIBWESTON_MAJOR@_la_CFLAGS = $(AM_CFLAGS) \
-	$(COMPOSITOR_CFLAGS) $(LIBUNWIND_CFLAGS) $(LIBDRM_CFLAGS)
+	$(COMPOSITOR_CFLAGS) $(EGL_CFLAGS) $(LIBUNWIND_CFLAGS) $(LIBDRM_CFLAGS)
 libweston_@LIBWESTON_MAJOR@_la_LIBADD = $(COMPOSITOR_LIBS) $(LIBUNWIND_LIBS) \
 	$(DLOPEN_LIBS) -lm $(CLOCK_GETTIME_LIBS) \
 	$(LIBINPUT_BACKEND_LIBS) libshared.la