Update inst_dir includes to use standard C headers

- Remove unnecessary <windows.h> include from inst_dir.c and inst_dir.h
- Add <stdlib.h> and <string.h> to inst_dir.c for calloc, free, strlen, memcpy
- Add <stddef.h> to inst_dir.h for size_t definition

Author: shinokaro

src/inst_dir.c

@@ -1,4 +1,5 @@
-#include <windows.h>
+#include <stdlib.h>
+#include <string.h>
 #include "error.h"
 #include "filesystem_utils.h"
 #include "inst_dir.h"

src/inst_dir.h

@@ -1,5 +1,5 @@
-#include <windows.h>
 #include <stdbool.h>
+#include <stddef.h>
 
 /**
  * @brief Creates an installation directory with a unique name.