mips: Fix the build for generic ANSI C89 compilers

The <stdint.h> header is a standard C99 header, and a MIPS-specific
header, but it is not a standard C89 header. It should not be included
until ensuring that the code being compiled is MIPS-specific.
diff --git a/mips/filter_msa_intrinsics.c b/mips/filter_msa_intrinsics.c
index a579179..0dab816 100644
--- a/mips/filter_msa_intrinsics.c
+++ b/mips/filter_msa_intrinsics.c
@@ -11,7 +11,6 @@
  */
 
 #include <stdio.h>
-#include <stdint.h>
 #include "../pngpriv.h"
 
 #ifdef PNG_READ_SUPPORTED
@@ -20,6 +19,7 @@
 #if PNG_MIPS_MSA_IMPLEMENTATION == 1 /* intrinsics code from pngpriv.h */
 
 #include <msa.h>
+#include <stdint.h>
 
 /* libpng row pointers are not necessarily aligned to any particular boundary,
  * however this code will only work with appropriate alignment. mips/mips_init.c