lib: fix compiler warnings

Fix the following warnings when building the tests by using the correct
types:
cast from 'const char *' to 'void *' drops const qualifier
[-Wcast-qual]
implicit conversion changes signedness [-Wsign-conversion]

lib/mprintf.c

@@ -610,7 +610,7 @@ static int dprintf_formatf(
     int is_neg;
 
     /* Base of a number to be written.  */
-    long base;
+    unsigned long base;
 
     /* Integral values to be written.  */
     mp_uintmax_t num;