Add libdrm_exynos_include_config in third_party/libdrm/BUILD.gn

We need it to be build libminigbm.so with use_exynos_backend=true

Bug: none
Change-Id: I35d9ee0971832eff51cca5fa40e386157c627a79
Reviewed-on: https://chromium-review.googlesource.com/957979Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542389}

third_party/libdrm/BUILD.gn

@@ -33,6 +33,10 @@ if (!use_system_libdrm) {
     }
   }
 
+  config("libdrm_exynos_include_config") {
+    include_dirs = [ "//third_party/libdrm/src/exynos" ]
+  }
+
   static_library("libdrm") {
     sources = [
       "src/xf86drm.c",

third_party/minigbm/BUILD.gn

@@ -99,6 +99,10 @@ if (!use_system_minigbm) {
     ]
     public_configs = [ ":minigbm_config" ]
 
+    if (use_exynos_minigbm) {
+      configs += [ "//third_party/libdrm:libdrm_exynos_include_config" ]
+    }
+
     # Clients need this to pick up the shared library correctly.
     all_dependent_configs =
         [ "//build/config/gcc:rpath_for_built_shared_libraries" ]