libstdc++: Fix typo in __size_to_integer(__GLIBCXX_TYPE_INT_N_3)

The overload taking a signed type was returning unsigned and the
overload taking an unsigned type was returning signed.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algobase.h (__size_to_integer): Move
	misplaced unsigned keyword on __size_to_integer overloads for
	__GLIBCXX_TYPE_INT_N_3 integer type.

(cherry picked from commit 106591f79a3beaed226400fbdc568e95229dc936)