alpha: fix fpu.h usage in userspace

After commit ec221208 ("Disintegrate asm/system.h for Alpha"), the
fpu.h header which we install for userland started depending on
special_insns.h which is not installed.

However, fpu.h only uses that for __KERNEL__ code, so protect the
inclusion the same way to avoid build breakage in glibc:

  /usr/include/asm/fpu.h:4:31: fatal error: asm/special_insns.h: No such file or directory

Cc: stable@vger.kernel.org
Reported-by: Matt Turner <mattst88@gentoo.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/alpha/include/asm/fpu.h

 #ifndef __ASM_ALPHA_FPU_H
 #define __ASM_ALPHA_FPU_H
 
+#ifdef __KERNEL__
 #include <asm/special_insns.h>
+#endif
 
 /*
  * Alpha floating-point control register defines: