Fix pkgconfig name for gtest

Summary:
Failed when trying to build folly. Apparently macos is not case
sensitive, but fedora is.

Reviewed By: bigfootjon

Differential Revision: D62470003

fbshipit-source-id: 885c76f6d44b10664090864314c0f2c13737ea0a

shim/third-party/googletest/BUCK

@@ -13,7 +13,7 @@ oncall("open_source")
 third_party_library(
     name = "gtest",
     homebrew_package_name = "googletest",
-    pkgconfig_name = "GTest",
+    pkgconfig_name = "gtest",
 )
 
 alias(