devel/gdb: declare dependency on Boost.Regex with SOURCE_HIGHLIGHT=ON (+)
Source-highlight forces consumers to link with liboost_regex.so
Without declared dependency gdb fails to start after boost update:
ld-elf.so.1: Shared object "libboost_regex.so.1.85.0" not found, required by "gdb"
Approved by: portmgr blanket
Sponsored by: Future Crew, LLC