ffmpeg: Fix pkg-config usage

http://crrev.com/c/1715466 blocks direct call of pkg-config,
use tc-getPKG_CONFIG to get pkg-config tool.

Since ffmpeg is uprev to new version and chromium version
of ebuild does not exist,so applied patch in upstream 9999
ebuild.
Cheery pick https://github.com/gentoo/gentoo/commit/bb91cfbdd67d5d5336de6d9d382f9ac9417b4cdc

BUG=chromium:985180
TEST=emerge-<board> ffmpeg

Change-Id: I3de4c37e0f560426b82c490c91eebc2611ca26cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2334238
Tested-by: Abhishek Kumar <abhishek4.kumar@intel.com>
Commit-Queue: Abhishek Kumar <abhishek4.kumar@intel.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/media-video/ffmpeg/ffmpeg-4.1.3.ebuild b/media-video/ffmpeg/ffmpeg-4.1.3.ebuild
index 8ab15e8..25c74da 100644
--- a/media-video/ffmpeg/ffmpeg-4.1.3.ebuild
+++ b/media-video/ffmpeg/ffmpeg-4.1.3.ebuild
@@ -452,6 +452,7 @@
 		--cc="$(tc-getCC)" \
 		--cxx="$(tc-getCXX)" \
 		--ar="$(tc-getAR)" \
+		--pkg-config="$(tc-getPKG_CONFIG)" \
 		--optflags="${CFLAGS}" \
 		$(use_enable static-libs static) \
 		"${myconf[@]}"