mtd: fix imagetag compilation

mtd: fix imagetag compilation

Commit b5b0796a1322 added an uint32_t to mtd.h without including stdint, which
results in a compilation error for those files not including stdint.h.

In file included from imagetag.c:36:
mtd.h:15:8: error: unknown type name 'uint32_t'
 extern uint32_t opt_trxmagic;
        ^~~~~~~~
imagetag.c: In function 'trx_fixup':
imagetag.c:180:10: warning: unused variable 'res' [-Wunused-variable]
  ssize_t res;
          ^~~
imagetag.c:177:14: warning: unused variable 'scan' [-Wunused-variable]
  void *ptr, *scan;
              ^~~~
imagetag.c: In function 'trx_check':
imagetag.c:246:27: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
     struct bcm_tag *tag = (const struct bcm_tag *) buf;
                           ^
make[3]: *** [<builtin>: imagetag.o] Error 1

Fixes: b5b0796a1322 ("mtd: add option for TRX magic to fixtrx")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>