rendervulkan: add missing header for libc++ 16 after 191a85d

In file included from src/drm.cpp:21:
In file included from src/drm.hpp:89:
src/rendervulkan.hpp:774:7: error: no type named 'mutex' in namespace 'std'
        std::mutex m_pipelineMutex;
        ~~~~~^

src/rendervulkan.hpp

@@ -9,6 +9,7 @@
 #include <unordered_map>
 #include <array>
 #include <bitset>
+#include <mutex>
 
 #include "main.hpp"