sim/aarch64: Fix aarch64_get_CPSR_bits() declaration

sim/aarch64: Fix aarch64_get_CPSR_bits() declaration

Noticed while doing mass builds with a very recent GCC:

/usr/lib/gcc-snapshot/bin/gcc  -DHAVE_CONFIG_H   -DWITH_HW=1 -DHAVE_DV_SOCKSER -DDEFAULT_INLINE=0 -Wall -Wdeclaration-after-statement -Wpointer-arith -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wno-error=maybe-uninitialized -Wmissing-declarations -Wmissing-prototypes -Wdeclaration-after-statement -Wmissing-parameter-type -Wpointer-sign -Wold-style-declaration -Werror  -I. -I/var/lib/laminar/run/gdb-aarch64-elf/1/binutils-gdb/sim/aarch64 -I../common -I/var/lib/laminar/run/gdb-aarch64-elf/1/binutils-gdb/sim/aarch64/../common -I../../include -I/var/lib/laminar/run/gdb-aarch64-elf/1/binutils-gdb/sim/aarch64/../../include -I../../bfd -I/var/lib/laminar/run/gdb-aarch64-elf/1/binutils-gdb/sim/aarch64/../../bfd -I../../opcodes -I/var/lib/laminar/run/gdb-aarch64-elf/1/binutils-gdb/sim/aarch64/../../opcodes -I../..  -I/var/lib/laminar/run/gdb-aarch64-elf/1/binutils-gdb/sim/aarch64/../../gnulib/import -I../../gnulib/import  -g -O2   -c -o cpustate.o -MT cpustate.o -MMD -MP -MF .deps/cpustate.Tpo cpustate.c
cpustate.c:270:1: error: conflicting types for 'aarch64_get_CPSR_bits' due to enum/integer mismatch; have 'uint32_t(sim_cpu *, FlagMask)' {aka 'unsigned int(struct _sim_cpu *, FlagMask)'} [-Werror=enum-int-mismatch]
  270 | aarch64_get_CPSR_bits (sim_cpu *cpu, FlagMask mask)
      | ^~~~~~~~~~~~~~~~~~~~~
In file included from sim-main.h:30,
                 from cpustate.c:28:
cpustate.h:310:20: note: previous declaration of 'aarch64_get_CPSR_bits' with type 'uint32_t(sim_cpu *, uint32_t)' {aka 'unsigned int(struct _sim_cpu *, unsigned int)'}
  310 | extern uint32_t    aarch64_get_CPSR_bits  (sim_cpu *, uint32_t);
      |                    ^~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors