picotcp: fix alloca on freebsd

core/deps/picotcp/include/arch/pico_posix.h

@@ -8,15 +8,17 @@
 
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <sys/time.h>
 // Note: alloca is not part of POSIX
+// find definition of alloca if it's not in stdlib.h:
 #if defined(_MSC_VER) || defined(__MINGW32__)
 #include <malloc.h>
-#else
+#endif
+#if defined(__linux__) || defined(__sun__)
 #include <alloca.h>
 #endif
-
 /*
    #define MEMORY_MEASURE
    #define TIME_PRESCALE