[OpenMP] Fix gettid warnings on DragonFly (#65549)

Define __kmp_gettid() as appropriate for DragonFly.

Author: brad0

openmp/runtime/src/kmp_wrapper_getpid.h

@@ -23,7 +23,7 @@
 #if KMP_OS_DARWIN
 // OS X
 #define __kmp_gettid() pthread_mach_thread_np(pthread_self())
-#elif KMP_OS_FREEBSD
+#elif KMP_OS_FREEBSD || KMP_OS_DRAGONFLY
 #include <pthread_np.h>
 #define __kmp_gettid() pthread_getthreadid_np()
 #elif KMP_OS_NETBSD