Fix __builtin_alloca_with_align_and_max defbuiltin usage

There is a missing space between the return type and the name
which causes the name not to be outputted in the html docs.

Committed as obvious after building html docs.

gcc/ChangeLog:

	* doc/extend.texi (__builtin_alloca_with_align_and_max): Fix
	defbuiltin usage.

(cherry picked from commit fb909bc1)