x86/msr: Fix build with older GCC versions

x86/msr: Fix build with older GCC versions

Some older GCC versions produce following error when x86 MSR modules are build:

  In file included from commands/i386/rdmsr.c:29:0:
  ../include/grub/i386/rdmsr.h:27:29: error: no previous prototype for ‘grub_msr_read’ [-Werror=missing-prototypes]
   extern inline grub_uint64_t grub_msr_read (grub_uint32_t msr_id)
                               ^
  cc1: all warnings being treated as errors

This happens due to lack of support for a such usage of extern keyword
in older GCCs. Additionally, this usage is not consistent with the rest
of codebase. So, replace it with static keyword.

Additionally, fix incorrect coding style.

Reported-by: Eric Snowberg <eric.snowberg@oracle.com>
Reported-by: adrian15 <adrian15sgd@gmail.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Vladimir 'phcoder' Serbinenko <phcoder@gmail.com>
Reviewed-by: Eric Snowberg <eric.snowberg@oracle.com>
Tested-by: adrian15 <adrian15sgd@gmail.com>