Provide embb_core_count_available() implementation for FreeBSD

  (this probably works on MacOS too, but not tested yet)

While on it, fix error message. The missing include file
is not the root cause of the problem. The missing implementation
of embb_core_count_available() is.

base_c/src/core_set.c

@@ -94,12 +94,22 @@ void embb_core_set_init(embb_core_set_t* core_set, int initializer) {
 #ifdef EMBB_HAS_HEADER_SYSINFO
 #include <sys/sysinfo.h>
 #endif
+#ifdef __FreeBSD__
+#include <sys/types.h>
+#include <sys/sysctl.h>
+#endif
 
 unsigned int embb_core_count_available() {
 #ifdef EMBB_HAS_HEADER_SYSINFO
   return get_nprocs();
+#elif defined __FreeBSD__
+  const size_t bs = sizeof(unsigned int);
+  char buf[bs];
+  size_t len = bs;
+  sysctlbyname("hw.ncpu", buf, &len, NULL, 0);
+  return *(unsigned int*)&buf;
 #else
-#error "No header sysinfo available!"
+#error "No implementation for embb_core_count_available()!"
 #endif
 }