thermald: use libmetrics.pc not libmetrics-${libbase_ver}.pc

libmetrics is now providing unversioned library. and versioned
pkg-config file will be removed.

BUG=chromium:920513
TEST=CQ

Change-Id: I72147574c785f60323ad8a75228ed718fc19dbb8
Reviewed-on: https://chrome-internal-review.googlesource.com/c/chromeos/thermald/+/3239583
Commit-Queue: Qijiang Fan <fqj@google.com>
Tested-by: Qijiang Fan <fqj@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index bb2cac8..f2d6dae 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -20,7 +20,7 @@
   pkg_deps = [
     "libbrillo",
     "libchrome-${libbase_ver}",
-    "libmetrics-${libbase_ver}",
+    "libmetrics",
   ]
 }