mips: ralink: add missing include

mips: ralink: add missing include

Missing headers causes an error on kernel 6.12:
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)
      |              ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/18654
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>