libstdc++: Add missing 'inline' to always_inline function

This fixes a -Wattributes warning for the COW std::string which was
previously suppressed due to being in a system header.

libstdc++-v3/ChangeLog:

	* include/bits/cow_string.h (__resize_for_overwrite): Add
	inline keyword to function with always_inline attribute.

(cherry picked from commit 48e1b89f14f5eab9eb3f61830f608d92c4ee54b6)
1 file changed