Fix Windows build: add missing #include <string> in test_ext.h

Summary:
D91606471 changed test_ext.h to take string_view, replacing #include <string> with #include <string_view>. However, the return type of test_find_resource_t is still std::string, which requires <string>. On GCC/Clang, <string_view> transitively includes <string>, so this went unnoticed on Linux/macOS. On MSVC (Windows), <string_view> does not transitively include <string>, causing the getdeps Windows CI build to fail with: error C2039: 'string': is not a member of 'std'.

This adds the missing #include <string> under an #ifdef _WIN32 guard to fix the Windows build.

Reviewed By: joanna-jo

Differential Revision: D92772665

fbshipit-source-id: 5d8b708ba97eae864bfae4641de7ec37b4e2f82a

Author: jbeshay

folly/ext/test_ext.h

@@ -16,6 +16,9 @@
 
 #pragma once
 
+#ifdef _WIN32
+#include <string>
+#endif
 #include <string_view>
 
 namespace folly {