core: fix libc++ compatibility (#75)

src/config/ConfigManager.cpp:91:6: error: no type named 'mutex' in namespace 'std'
   91 | std::mutex   configMtx;
      | ~~~~~^
src/config/ConfigManager.cpp:94:10: error: no member named 'lock_guard' in namespace 'std'
   94 |     std::lock_guard<std::mutex> lg(configMtx);
      |     ~~~~~^
/var/tmp/portage/gui-apps/hyprlock-9999/work/hyprlock-9999/src/config/ConfigManager.cpp:94:26: error: no member named 'mutex' in namespace 'std'
   94 |     std::lock_guard<std::mutex> lg(configMtx);
      |                     ~~~~~^

Closes: #43

Signed-off-by: Gonçalo Negrier Duarte <[email protected]>

src/config/ConfigManager.cpp

@@ -3,6 +3,7 @@
 #include <filesystem>
 #include <glob.h>
 #include <cstring>
+#include <mutex>
 
 static Hyprlang::CParseResult handleSource(const char* c, const char* v) {
     const std::string      VALUE   = v;
@@ -211,4 +212,4 @@ std::optional<std::string> CConfigManager::handleSource(const std::string& comma
     }
 
     return {};
-}
+}