Fix undefined mips_use_msa identifier in third_party/BUILD.gn.

Add the missing //build/config/mips.gni import.

Bug: pdfium:1626
Change-Id: I1424152ba2c13906a1462b6fa6ae6c9a365c95d4
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/79535
Reviewed-by: Hui Yingst <nigi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index f73baa5..2e1ed11 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//build/config/arm.gni")
 import("//build/config/linux/pkg_config.gni")
+import("//build/config/mips.gni")
 import("//build_overrides/build.gni")
 import("../pdfium.gni")