string.h: remove unused variable

Change-Id: I08b7dab1b374ad427fcbee4183f2c7d93d0199a9
diff --git a/libc/include/string.h b/libc/include/string.h
index b0643af..f3dca1b 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -117,7 +117,6 @@
 void* memcpy(void* __restrict dest, const void* __restrict src, size_t copy_amount) {
     char *d = (char *) dest;
     const char *s = (const char *) src;
-    size_t s_len = __bos0(s);
     size_t d_len = __bos0(d);
 
     return __builtin___memcpy_chk(dest, src, copy_amount, d_len);