memset(): Make it clear 'c' is intepreted as 'unsigned char'

No change in behavior due to existing implicit cast, but make it clear
that the character to set is interpreted as 'unsigned char', as per C99.

lib/libxc/memset.c

@@ -23,11 +23,12 @@
 void *memset(void *s, int c, size_t n)
 {
     unsigned char *p = s;
+    unsigned char byte = c;
     size_t i;
 
     for (i = 0; i < n; i++)
     {
-        p[i] = c;
+        p[i] = byte;
     }
     return s;
 }