Add missing include for va_copy

See https://bugs.gentoo.org/929232

clang -c -o ../tmp/wip/rss/20stringvformat.o -O3 -pipe -march=native -fno-diagnostics-color -Werror=implicit-function-declaration -Werror=implicit-int -c stringvformat.c
stringvformat.c:13:5: error: array type 'va_list' (aka '__builtin_va_list') is
      not assignable
   13 |     va_copy(copy, list);