minigbm: Install minigbm_helpers.h

Install the header for others to use it.

BUG=b:175246639, b:240390351
TEST=emerge-zork minigbm

Change-Id: Ib6cc8055aaace23132634c2478ae8c586340e806
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/3780065
Reviewed-by: Dominik Behr <dbehr@chromium.org>
Commit-Queue: Chung-Sheng Wu <chungsheng@google.com>
Auto-Submit: Chung-Sheng Wu <chungsheng@google.com>
Tested-by: Chung-Sheng Wu <chungsheng@google.com>
diff --git a/Makefile b/Makefile
index 987708f..b45226d 100644
--- a/Makefile
+++ b/Makefile
@@ -59,3 +59,4 @@
 	ln -sf $(MINIGBM_FILENAME) $(DESTDIR)/$(LIBDIR)/libgbm.so.$(GBM_VERSION_MAJOR)
 	install -D -m 0644 $(SRC)/gbm.pc $(DESTDIR)$(LIBDIR)/pkgconfig/gbm.pc
 	install -D -m 0644 $(SRC)/gbm.h $(DESTDIR)/usr/include/gbm.h
+	install -D -m 0644 $(SRC)/minigbm_helpers.h $(DESTDIR)/usr/include/minigbm/minigbm_helpers.h