Add nowarnings in gdb.base/fileio.exp

Add nowarnings in gdb.base/fileio.exp

I see the following warning in gdb.base/fileio.c,

testsuite/gdb.base/fileio.c:297:3: warning: null argument where non-null required (argument 1) [-Wnonnull]
   ret = stat (NULL, &st);
   ^

This patch adds "nowarnings" to the list passed to gdb_compile.

gdb/testsuite:

2017-05-17  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/fileio.exp: Pass nowarnings to gdb_compile.