MIPS: launch.h: add include guard to prevent build errors

arch/mips/include/asm/mips-boards/launch.h needs an include guard
to prevent it from being #included more than once.
Prevents these build errors:

In file included from ../arch/mips/mti-malta/malta-amon.c:16:
../arch/mips/include/asm/mips-boards/launch.h:8:8: error: redefinition of 'struct cpulaunch'
8 | struct cpulaunch {
| ^~~~~~~~~
In file included from ../arch/mips/include/asm/mips-cps.h:13,
from ../arch/mips/include/asm/smp-ops.h:16,
from ../arch/mips/include/asm/smp.h:21,
from ../include/linux/smp.h:114,
from ../arch/mips/mti-malta/malta-amon.c:12:
../arch/mips/include/asm/mips-boards/launch.h:8:8: note: originally defined here
8 | struct cpulaunch {
| ^~~~~~~~~
make[3]: [../scripts/Makefile.build:273: arch/mips/mti-malta/malta-amon.o] Error 1 (ignored)

Fixes: 6decd1aad15f ("MIPS: add support for buggy MT7621S core detection")
Signed-off-by: Randy Dunlap <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: [e-mail]
Cc: Ilya Lipnitskiy <[e-mail]>
Reviewed-by: Ilya Lipnitskiy <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/include/asm/mips-boards/launch.h

···
  *
  */
 
+#ifndef _ASM_MIPS_BOARDS_LAUNCH_H
+#define _ASM_MIPS_BOARDS_LAUNCH_H
+
 #ifndef _ASSEMBLER_
 
 struct cpulaunch {
···
 
 /* Polling period in count cycles for secondary CPU's */
 #define LAUNCHPERIOD	10000
+
+#endif /* _ASM_MIPS_BOARDS_LAUNCH_H */