minimp3: move header include guard before implementation. Add implementation guard.
minimp3: move header include guard before implementation. Add implementation guard.

Test cases:
1.c
 #include "minimp3.h" /* gives definitions */
 ...
 #define MINIMP3_IMPLEMENTATION
 #include "minimp3.h" /* gives implementation */

2.c
 #define MINIMP3_IMPLEMENTATION
 #include "minimp3.h" /* gives defintions and implementation */
 #include "minimp3.h" /* does not gives anything to not confuse compiler */

lieff/minimp3,lieff/minimp3,lieff/minimp3