Move declaration of ast_regex_string_to_regex_pattern futher down strings.h.

The prior location is before the declaration of struct ast_str, which causes
compiler warnings.

(closes issue ASTERISK-20852)
Reported by: Pavel Troller
Patches:
	strings.diff uploaded by Pavel Troller (license 6302)
........

Merged revisions 378747 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378748 65c4cc65-6c06-0410-ace0-fbb531ad65f3