coap_config_posix.h: Set up defines for correct mutex usage

if COAP_CONSTRAINED_STACK is defined (is for esp-idf), the mutexes set up
to protect the critical sections were not working as expected and were just
dummy entries.

Use the pthread_ version of the mutex instead by defining HAVE_PTHREAD_H
and HAVE_PTHREAD_MUTEX_LOCK.

components/coap/port/include/coap_config_posix.h

@@ -30,6 +30,8 @@
 #define HAVE_NETDB_H
 #define HAVE_NETINET_IN_H
 #define HAVE_STRUCT_CMSGHDR
+#define HAVE_PTHREAD_H
+#define HAVE_PTHREAD_MUTEX_LOCK
 #define COAP_DISABLE_TCP 0
 
 #define ipi_spec_dst ipi_addr