prf.c: don't be silent with unknown conversion specifiers

Mimic the glibc behavior when encountering an unknown conversion
specifier rather than silently skipping it.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

lib/libc/minimal/source/stdout/prf.c

@@ -737,6 +737,12 @@ int z_prf(int (*func)(), void *dest, const char *format, va_list vargs)
 				count++;
 				break;
 
+			default:
+				PUTC('%');
+				PUTC(c);
+				count += 2;
+				break;
+
 			case 0:
 				return count;
 			}