rtpengine: use perl/host when building

rtpengine: use perl/host when building

utils/const_str_hash is called by rtpengine build system. The Perl
script uses shebang "#!/usr/bin/perl" and depends on perl-IPC-Cmd. If
this is missing the build will fail:

../utils/const_str_hash < "control_ng.c" > "control_ng.strhash.c"
Can't locate IPC/Cmd.pm in @INC (you may need to install the IPC::Cmd module) (@INC contains: /usr/local/lib64/perl5/5.34 /usr/local/share/perl5/5.34 /usr/lib64/perl5/vendor_perl /usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5) at ../utils/const_str_hash line 6.
BEGIN failed--compilation aborted at ../utils/const_str_hash line 6.
make[4]: *** [../lib/common.Makefile:46: control_ng.strhash.c] Error 2

OpenWrt does not depend on perl-IPC-Cmd being available, so this commit
makes rtpengine build depend on perl/host, which provides this Perl
module.

Closes #748

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>