PipeWire capturer: search for epoxy headers

We actually use headers from libepoxy (it's part of the sysroot) so this
was removed accicentally in one of previous changes and it just
magically worked as we include those headers with their full path

Bug: webrtc:13429
Change-Id: I4f5684521a76287a725272ce3833daae673d9332
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/252002
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#36073}

modules/desktop_capture/BUILD.gn

@@ -228,6 +228,10 @@ if (is_linux || is_chromeos) {
     pkg_config("egl") {
       packages = [ "egl" ]
     }
+    pkg_config("epoxy") {
+      packages = [ "epoxy" ]
+      ignore_libs = true
+    }
     pkg_config("libdrm") {
       packages = [ "libdrm" ]
       if (!rtc_link_pipewire) {
@@ -577,6 +581,7 @@ rtc_library("desktop_capture_generic") {
       ":pipewire",
       ":gbm",
       ":egl",
+      ":epoxy",
       ":libdrm",
     ]