kill extern of vsyscall32_sysctl

it's been dead for years.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/x86/include/asm/elf.h

@@ -226,7 +226,6 @@ do {								\
 /* I'm not sure if we can use '-' here */
 #define ELF_PLATFORM       ("x86_64")
 extern void set_personality_64bit(void);
-extern unsigned int sysctl_vsyscall32;
 extern int force_personality32;
 
 #endif /* !CONFIG_X86_32 */