MIPS: ralink: add missing header include

MIPS: ralink: add missing header include

Add the missing header "asm/time.h" which defines the function
prototypes of get_c0_perfcount_int() and get_c0_compare_int().
This patch fixes the following build warnings:

arch/mips/ralink/irq.c:86:5: error: no previous prototype for 'get_c0_perfcount_int' [-Werror=missing-prototypes]
   86 | int get_c0_perfcount_int(void)
      |     ^~~~~~~~~~~~~~~~~~~~
arch/mips/ralink/irq.c:92:14: error: no previous prototype for 'get_c0_compare_int' [-Werror=missing-prototypes]
   92 | unsigned int get_c0_compare_int(void)
      |              ^~~~~~~~~~~~~~~~~~

Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>