telephony/kamailio: build fix for upcoming LibreSSL pc change

With an upcoming change to the LibreSSL pc files (for OpenBSD only), the
output of 'pkg-config libssl --modversion' becomes 2.0.0 instead of 1.0.0.
This results in LIBSSL_SET_MUTEX_SHARED = 1 being set in src/Makefile.defs,
which in turn ends up adding LIBDL=-ldl to the linker flags and that breaks
the build.

The below forces LIBSSL_SET_MUTEX_SHARED back to 0, the value that has
been used up to now.

Another option would be to modify Makefile.defs to treat OpenBSD
as NetBSD and set LIBDL="" to avoid -ldl. However, this would
enable some workaround for "libssl 1.1+ thread-only mutex
initialization" which wasn't previously needed (see end of src/main.c).

Index: Makefile
===================================================================
RCS file: /cvs/ports/telephony/kamailio/Makefile,v
retrieving revision 1.50
diff -u -p -r1.50 Makefile
--- Makefile 4 Dec 2021 21:22:58 -0000 1.50
+++ Makefile 3 Feb 2022 14:28:32 -0000
@@ -69,7 +69,8 @@ MAKE_FLAGS = LIBDIR=lib \
group_include="${KAMAILIO_GROUPS}" \
LIBsrdb1_VERSION=${LIBsrdb1_VERSION} \
LIBsrdb2_VERSION=${LIBsrdb2_VERSION} \
- LIBtrie_VERSION=${LIBtrie_VERSION}
+ LIBtrie_VERSION=${LIBtrie_VERSION} \
+ LIBSSL_SET_MUTEX_SHARED=0

PSEUDO_FLAVORS += no_berkeley
.if !${FLAVOR:Mno_berkeley}