Fix building with musl libc

POSIX says poll.h and signal.h should not be prefixed with sys/ (ie
poll.h instead of sys/poll.h)

limits.h also defines a TZNAME_MAX so we change name of the define for
spandsp.

src/mod/applications/mod_spandsp/mod_spandsp_modem.h

@@ -89,7 +89,7 @@
 	(uint64_t)bswap_32((uint32_t)((value) >> 32)))
 #endif
 #include <sys/time.h>
-#include <sys/signal.h>
+#include <signal.h>
 #endif
 #include <sys/types.h>
 #include <sys/stat.h>