Use '::iterator' instead of '::const_iterator' on environ.c (and fix breakage on early versions of libstdc++)

Even though C++11 supports modifying containers using a const_iterator
(e.g., calling the 'erase' method of a std::vector), early versions of
libstdc++ did not implement that.  Some of our buildslaves are using
these versions (e.g., the AArch64 buildslave uses gcc 4.8.8), and my
previous commit causes a breakage on them.  The solution is simple:
just use a normal iterator, without const.

gdb/ChangeLog:
2017-06-20  Sergio Durigan Junior  <sergiodj@redhat.com>

	* common/environ.c (gdb_environ::unset): Use '::iterator' instead
	of '::const_iterator'.

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