powerpc: Only define __parse_fpscr() when required

Clang 17 reports:

arch/powerpc/kernel/traps.c:1167:19: error: unused function '__parse_fpscr' [-Werror,-Wunused-function]

__parse_fpscr() is called from two sites. First call is guarded
by #ifdef CONFIG_PPC_FPU_REGS

Second call is guarded by CONFIG_MATH_EMULATION which selects
CONFIG_PPC_FPU_REGS.

So only define __parse_fpscr() when CONFIG_PPC_FPU_REGS is defined.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309210327.WkqSd5Bq-lkp@intel.com/
Fixes: b6254ced ("powerpc/signal: Don't manage floating point regs when no FPU")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/5de2998c57f3983563b27b39228ea9a7229d4110.1695385984.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/traps.c

@@ -1164,6 +1164,7 @@ void emulate_single_step(struct pt_regs *regs)
 		__single_step_exception(regs);
 }
 
+#ifdef CONFIG_PPC_FPU_REGS
 static inline int __parse_fpscr(unsigned long fpscr)
 {
 	int ret = FPE_FLTUNK;
@@ -1190,6 +1191,7 @@ static inline int __parse_fpscr(unsigned long fpscr)
 
 	return ret;
 }
+#endif
 
 static void parse_fpe(struct pt_regs *regs)
 {