add include of stdio.h, so that base64.c compiles

include/curl/mprintf.h

@@ -54,6 +54,7 @@
 #define H_MPRINTF
 
 #include <stdarg.h>
+#include <stdio.h> /* needed for FILE */
 
 int curl_mprintf(const char *format, ...);
 int curl_mfprintf(FILE *fd, const char *format, ...);