update fboss gtest usage for cmake 3.20+ and centos stream 8

Summary:
Need to specify both gtest and gmock includes like folly does

Centos stream 8 doesn't have libmnl-static, and needed different alias for nl-route-3

Reviewed By: shri-khare

Differential Revision: D33537568

fbshipit-source-id: 506ef13224e71b874f8a43b52474f39da14a8d67

build/fbcode_builder/manifests/libmnl

@@ -3,6 +3,9 @@ name = libmnl
 
 [rpms]
 libmnl-devel
+
+# centos stream 8 is missing this, but its in fedora so may be back in a later version
+[rpms.not(all(distro=centos_stream,distro_vers=8))]
 libmnl-static
 
 [debs]