FreeBSD libzfs: gcc requires __thread after static

Building libzfs with gcc on FreeBSD failed because gcc is picky about
the order of keywords in declarations with __thread, whereas clang is
more relaxed.

https://gcc.gnu.org/onlinedocs/gcc/Thread-Local.html

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Closes #11331

lib/libzfs/os/freebsd/libzfs_compat.c

@@ -178,7 +178,7 @@ execvpe(const char *name, char * const argv[], char * const envp[])
 
 #define	ERRBUFLEN 256
 
-__thread static char errbuf[ERRBUFLEN];
+static __thread char errbuf[ERRBUFLEN];
 
 const char *
 libzfs_error_init(int error)