minor: Fix mongoose build without threads (#2291)

Fix the following build failure without threads:

In file included from /home/buildroot/autobuild/instance-3/output-1/build/rtl_433-22.11/src/data_tag.c:17:
/home/buildroot/autobuild/instance-3/output-1/build/rtl_433-22.11/include/mongoose.h:407:10: fatal error: pthread.h: No such file or directory
  407 | #include <pthread.h>
      |          ^~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/9f1677fc5a4568be0b9c121060c5d821ac2ae21b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

include/mongoose.h

@@ -404,7 +404,6 @@ unsigned int sleep(unsigned int seconds);
 #include <math.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#include <pthread.h>
 #include <signal.h>
 #include <stdarg.h>
 #include <stdbool.h>
@@ -4658,6 +4657,7 @@ size_t mg_fwrite(const void *ptr, size_t size, size_t count, FILE *f);
 #endif /* MG_ENABLE_FILESYSTEM */
 
 #if MG_ENABLE_THREADS
+#include <pthread.h>
 /*
  * Starts a new detached thread.
  * Arguments and semantics are the same as pthead's `pthread_create()`.