kaiser: fix compile error without vsyscall

BugLink: http://bugs.launchpad.net/bugs/1756121

Tobias noticed a compile error on 4.4.115, and it's the same on 4.9.80:
arch/x86/mm/kaiser.c: In function ‘kaiser_init’:
arch/x86/mm/kaiser.c:348:8: error: ‘vsyscall_pgprot’ undeclared
                                   (first use in this function)

It seems like his combination of kernel options doesn't work for KAISER.
X86_VSYSCALL_EMULATION is not set on his system, while LEGACY_VSYSCALL
is set to NONE (LEGACY_VSYSCALL_NONE=y). He managed to get things
compiling again, by moving the 'extern unsigned long vsyscall_pgprot'
outside of the preprocessor statement. This works because the optimizer
removes that code (vsyscall_enabled() is always false) - and that's how
it was done in some older backports.
Reported-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/include/asm/vsyscall.h

@@ -13,7 +13,6 @@ extern void map_vsyscall(void);
  */
 extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);
 extern bool vsyscall_enabled(void);
-extern unsigned long vsyscall_pgprot;
 #else
 static inline void map_vsyscall(void) {}
 static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
@@ -22,5 +21,6 @@ static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 }
 static inline bool vsyscall_enabled(void) { return false; }
 #endif
+extern unsigned long vsyscall_pgprot;
 
 #endif /* _ASM_X86_VSYSCALL_H */