Add namespace std to nullptr_t

Add namespace std to nullptr_t

This patch fixes the build failure for target i686-w64-mingw32,

In file included from ../../binutils-gdb/gdb/defs.h:786:0,
                 from ../../binutils-gdb/gdb/gdb.c:19:
../../binutils-gdb/gdb/utils.h:188:20: error: ‘nullptr_t’ has not been declared
   bool operator!= (nullptr_t)
                    ^
../../binutils-gdb/gdb/utils.h:193:20: error: ‘nullptr_t’ has not been declared
   bool operator== (nullptr_t)
                    ^

gdb:

2017-08-04  Yao Qi  <yao.qi@linaro.org>

	* utils.h (gdb_argv): Add namespace std for nullptr_t.