libosmovty: Link libosmovty against libpthread

Ever since Change-Id If76a4bd2cc7b3c7adf5d84790a944d78be70e10a in 2020
(part of libosmocore >= 1.4.0) we have introduced cpu_sched_vty.c, which
directly uses libpthread.  As a result, libosmovty should be using
pthread compiler flags and link against libpthread.

This missing dependency is causing osmocom applications to
fail to link on OpenWRT (at leats for ath79-generic).

Change-Id: I7febbf88cbe61eacd05f46a9316e773b5c148e77
Related: SYS#4986
diff --git a/src/vty/Makefile.am b/src/vty/Makefile.am
index c73dbb2..37faad6 100644
--- a/src/vty/Makefile.am
+++ b/src/vty/Makefile.am
@@ -4,7 +4,7 @@
 LIBVERSION=10:0:1
 
 AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_builddir)/include
-AM_CFLAGS = -Wall $(TALLOC_CFLAGS)
+AM_CFLAGS = -Wall $(TALLOC_CFLAGS) $(PTHREAD_CFLAGS)
 
 if ENABLE_VTY
 lib_LTLIBRARIES = libosmovty.la
@@ -14,5 +14,5 @@
 			fsm_vty.c talloc_ctx_vty.c \
 			cpu_sched_vty.c tdef_vty.c
 libosmovty_la_LDFLAGS = -version-info $(LIBVERSION) -no-undefined
-libosmovty_la_LIBADD = $(top_builddir)/src/libosmocore.la $(TALLOC_LIBS)
+libosmovty_la_LIBADD = $(top_builddir)/src/libosmocore.la $(TALLOC_LIBS) $(PTHREAD_LIBS)
 endif