BUILD: Makefile: enable USE_SHM_OPEN by default on freebsd

The shm_open() feature seems to work on freebsd, let's enable it by
default on the freebsd target.
diff --git a/Makefile b/Makefile
index 953c0ea..cfd9ede 100644
--- a/Makefile
+++ b/Makefile
@@ -373,7 +373,7 @@
 ifeq ($(TARGET),freebsd)
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_TPROXY USE_LIBCRYPT USE_THREAD USE_CPU_AFFINITY USE_KQUEUE   \
-    USE_ACCEPT4 USE_CLOSEFROM USE_GETADDRINFO USE_PROCCTL)
+    USE_ACCEPT4 USE_CLOSEFROM USE_GETADDRINFO USE_PROCCTL USE_SHM_OPEN)
 endif
 
 # kFreeBSD glibc