1030: Wrap get_search_region_from_search_nodes in #ifdef HAVE_REGEX_H

1030: Wrap get_search_region_from_search_nodes in #ifdef HAVE_REGEX_H

This change avoids the following error:

gcc -DHAVE_CONFIG_H -I../../.. -I/home/Kalle/src/elinks-0.11/src -I/home/Kalle/prefix/include -I/usr/include/smjs -I/usr/include -I/usr/include/lua50 -I/usr/include -I/usr/include -O0 -ggdb -Wall -Wall -Werror -fno-strict-aliasing -Wno-pointer-sign -Wno-address -fno-strict-overflow -o search.o -c /home/Kalle/src/elinks-0.11/src/viewer/text/search.c
cc1: warnings being treated as errors
/home/Kalle/src/elinks-0.11/src/viewer/text/search.c:257: warning: 'get_search_region_from_search_nodes' defined but not used
make[3]: *** [search.o] Error 1
make[3]: Leaving directory `/home/Kalle/build/i686-pc-linux-gnu/elinks-0.11/src/viewer/text'

get_search_region_from_search_nodes is called only from
search_for_pattern, which already was inside #ifdef HAVE_REGEX_H.