openvr: Fix a -Wmicrosoft-cast warning.

Standard C++ doesn't allow implicit conversion between function pointers
and void*. MSVC does allow that, so clang-cl also allows it but emits a
-Wmicrosoft-cast warning. We want to enable this warning to make the
compiler behave more similar on different platforms, so add an explicit
cast to void*. (GetProcAddress() returns FARPROC, a function pointer type.)

Merges one line from upstream 5d0574bf6473130d25dd296ad30206ccd148590b

Bug: 550065
Change-Id: I54080e807e2106ff084adc6e0f7954a72bdf467f
Reviewed-on: https://chromium-review.googlesource.com/953722Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541776}

third_party/openvr/src/src/vrcommon/sharedlibtools_public.cpp

@@ -22,7 +22,7 @@ SharedLibHandle SharedLib_Load( const char *pchPath )
 void *SharedLib_GetFunction( SharedLibHandle lib, const char *pchFunctionName)
 {
 #if defined( _WIN32)
-	return GetProcAddress( (HMODULE)lib, pchFunctionName );
+	return (void*)GetProcAddress( (HMODULE)lib, pchFunctionName );
 #elif defined(POSIX)
 	return dlsym( lib, pchFunctionName );
 #endif