upnp: force -lpthread (fixes #24239)

The pkg-config file correctly supplies -pthread for LDFLAGS, but
libtool discards it, leading to linkage failure if libupnp is linked
statically.

modules/services_discovery/Makefile.am

@@ -40,6 +40,11 @@ sd_LTLIBRARIES += $(LTLIBupnp)
 if HAVE_OSX
 libupnp_plugin_la_LDFLAGS += -Wl,-framework,CoreFoundation,-framework,SystemConfiguration
 endif
+if HAVE_LINUX
+if !HAVE_ANDROID
+libupnp_plugin_la_LIBADD += -lpthread
+endif
+endif
 
 libpulselist_plugin_la_SOURCES = services_discovery/pulse.c
 libpulselist_plugin_la_CFLAGS = $(AM_CFLAGS) $(PULSE_CFLAGS)