Common: Move header guards over to pragma once

Also replaced C headers with the C++ equivalent ones

src/common/std_condition_variable.h

@@ -1,6 +1,5 @@
 
-#ifndef CONDITION_VARIABLE_H_
-#define CONDITION_VARIABLE_H_
+#pragma once
 
 #define GCC_VER(x,y,z)    ((x) * 10000 + (y) * 100 + (z))
 #define GCC_VERSION GCC_VER(__GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__)
@@ -167,4 +166,3 @@ private:
 }
 
 #endif
-#endif