rename pc file from libgbm.pc to gbm.pc to match mesa

BUG=none
TEST=emerge-$BOARD minigbm

Change-Id: I640b12bd31b7f93287600a636324adc8f4432dca
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/229186
diff --git a/Makefile b/Makefile
index b330f81..d33d278 100644
--- a/Makefile
+++ b/Makefile
@@ -35,5 +35,5 @@
 install: all
 	mkdir -p $(DESTDIR)/$(LIBDIR)
 	install -D -m 755 $(OUT)/libgbm.so $(DESTDIR)/$(LIBDIR)
-	install -D -m 0644 $(SRC)/libgbm.pc $(DESTDIR)$(LIBDIR)/pkgconfig/libgbm.pc
+	install -D -m 0644 $(SRC)/gbm.pc $(DESTDIR)$(LIBDIR)/pkgconfig/gbm.pc
 	install -D -m 0644 $(SRC)/gbm.h $(DESTDIR)/usr/include/gbm.h