vc4: automake: remove vc4_drm.h from the sources lists

The file was removed with earlier commit breaking 'make dist'.
Drop it from Makefile.sources since it's no longer around.

Fixes: 16985eb308 ("vc4: Switch to using the libdrm-provided
vc4_drm.h.")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

src/gallium/drivers/vc4/Makefile.sources

@@ -14,7 +14,6 @@ C_SOURCES := \
 	vc4_context.c \
 	vc4_context.h \
 	vc4_draw.c \
-	vc4_drm.h \
 	vc4_emit.c \
 	vc4_fence.c \
 	vc4_formats.c \