gbm: split out source file to Makefile.sources

In preparation to add Android build support, split out the source file
lists to Makefile.sources

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

[Emil Velikov: Whitespace cleanup.]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/gbm/Makefile.sources

@@ -0,0 +1,11 @@
+gbm_core_FILES = \
+	main/backend.c \
+	main/backend.h \
+	main/common_drm.h \
+	main/gbm.c \
+	main/gbm.h \
+	main/gbmint.h
+
+gbm_dri_FILES = \
+	backends/dri/gbm_dri.c \
+	backends/dri/gbm_driint.h