gbm: do not build intermittent libgbm_dri static library

The only user of it (libgbm.la) immediately links it. Just build it
directly into the library.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/gbm/Makefile.am

 endif
 
 if HAVE_DRI2
-noinst_LTLIBRARIES = libgbm_dri.la
-libgbm_dri_la_SOURCES = \
+libgbm_la_SOURCES += \
 	backends/dri/gbm_dri.c \
 	backends/dri/gbm_driint.h
 
-libgbm_dri_la_CFLAGS = \
-	$(AM_CFLAGS) \
+AM_CFLAGS += \
 	-DDEFAULT_DRIVER_DIR='"$(DRI_DRIVER_SEARCH_DIR)"' \
 	$(LIBDRM_CFLAGS)
 
 libgbm_la_LIBADD += \
-	libgbm_dri.la \
 	$(LIBDRM_LIBS)
 endif