um: link with -lpthread

Similarly to commit fb1770aa78, with gcc 5
on Ubuntu and CONFIG_STATIC_LINK=y I was seeing these linker errors:

/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/librt.a(timer_create.o): In function `__timer_create_new':
(.text+0xcd): undefined reference to `pthread_once'
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/librt.a(timer_create.o): In function `__timer_create_new':
(.text+0x126): undefined reference to `pthread_attr_init'
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/librt.a(timer_create.o): In function `__timer_create_new':
(.text+0x168): undefined reference to `pthread_attr_setdetachstate'
[...]

Obviously we also need -lpthread for librt.a.

Cc: stable@vger.kernel.org # 4.4
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

scripts/link-vmlinux.sh

@@ -62,7 +62,7 @@ vmlinux_link()
 			-Wl,--start-group                                    \
 				 ${KBUILD_VMLINUX_MAIN}                      \
 			-Wl,--end-group                                      \
-			-lutil -lrt ${1}
+			-lutil -lrt -lpthread ${1}
 		rm -f linux
 	fi
 }