CMake: Only add postfixes if MSVC

Currently, it catches mingw-w64 in there as well, but mingw-w64 follows
linux-like naming with no weird postfixes

Signed-off-by: Christopher Degawa <ccom@randomderp.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d6ee9ec..a437717 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -496,7 +496,7 @@
 	VERSION ${PROJECT_VERSION}
 )
 
-if(WIN32)
+if(MSVC)
 	if(BUILD_SHARED_LIBS)
 		set_target_properties(
 			LibXml2