ftp/weex: fix build on powerpc

strlib.c:176:2: error: array type 'va_list' (aka '__builtin_va_list') is not assignable

176 |         G_VA_COPY(ap2,ap1);
    |         ^~~~~~~~~~~~~~~~~~

strlib.c:48:42: note: expanded from macro 'G_VA_COPY'

48 | #  define G_VA_COPY(ap1, ap2)     ((ap1) = (ap2))
   |                                    ~~~~~ ^

1 error generated.