Require libdrm only if use_ozone=1.

This is more accurate, and doesn't break platforms with no pkgconfig,
e.g. Android.

BUG=angleproject:1297
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_gn_chromeos_rel;tryserver.chromium.android:android_compile_dbg

Change-Id: Iafbd66a5e81467d2a5431c3bb1ddd26a837888e7
Reviewed-on: https://chromium-review.googlesource.com/356481
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 9637545..0fe679d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,8 +8,10 @@
 import("//build/config/ui.gni")
 import("//third_party/angle/build/angle_common.gni")
 
-pkg_config("libdrm") {
-  packages = [ "libdrm" ]
+if (use_ozone) {
+  pkg_config("libdrm") {
+    packages = [ "libdrm" ]
+  }
 }
 
 angle_git_is_present = exec_script("src/commit_id.py",