Include unistd.h before cheching _POSIX_MAPPED_FILES

Author: hasumikin

include/prism/util/pm_string.h

@@ -13,6 +13,7 @@
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 // The following headers are necessary to read files using demand paging.
 #ifdef _WIN32
@@ -21,7 +22,6 @@
 #include <fcntl.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
-#include <unistd.h>
 #endif
 
 /**