Follow-up to earlephilhower#1795 (earlephilhower#2177)

Address the review comment to fix conditional compilation syntax:
raspberrypi/pico-sdk#1795 (review)

Author: gneverov

src/rp2_common/pico_clib_interface/picolibc_interface.c

@@ -39,8 +39,9 @@ static int picolibc_putc(char c, __unused FILE *file) {
 static int picolibc_getc(__unused FILE *file) {
 #if LIB_PICO_STDIO
     return stdio_getchar();
-#endif
+#else
     return -1;
+#endif
 }
 
 static int picolibc_flush(__unused FILE *file) {