Rename util.c to string.c and introduce string.h.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/bmp.c b/src/bmp.c
index 0d54efd..81be306 100644
--- a/src/bmp.c
+++ b/src/bmp.c
@@ -6,8 +6,9 @@
 *
 * This work is licensed under the terms of the GNU LGPLv3.
 */
-#include "util.h"
-#include "bmp.h"
+#include "util.h" // malloc_tmphigh
+#include "bmp.h" // struct bmp_decdata
+#include "string.h" // memcpy
 
 #define bmp_load4byte(addr) (*(u32 *)(addr))
 #define bmp_load2byte(addr) (*(u16 *)(addr))