virtual_buffer: Mark size parameter of FreeMemoryPages() as [[maybe_unused]]

This isn't used on Windows, but is used on non-Windows operating
systems.

src/common/virtual_buffer.cpp

@@ -38,7 +38,7 @@ void* AllocateMemoryPages(std::size_t size) {
     return base;
 }
 
-void FreeMemoryPages(void* base, std::size_t size) {
+void FreeMemoryPages(void* base, [[maybe_unused]] std::size_t size) {
     if (!base) {
         return;
     }