Add pthread to Windows allowed ldlibs

The MinGW update now includes pthread.

Test: update BoringSSL to use pthreads on Windows and compile
Change-Id: Id85d9fd3e03a310e07e0093ca80cefaf10fe8930
diff --git a/cc/config/x86_windows_host.go b/cc/config/x86_windows_host.go
index 125d02d..79c9e36 100644
--- a/cc/config/x86_windows_host.go
+++ b/cc/config/x86_windows_host.go
@@ -82,6 +82,7 @@
 		"imagehlp",
 		"ole32",
 		"psapi",
+		"pthread",
 		"userenv",
 		"uuid",
 		"ws2_32",