soc/intel/common: Fix build error in reset.c

Fix build error caused by macro substitution in the function definition
when the Kconfig value HAVE_HARD_RESET is not selected.

src/soc/intel/common/reset.c:36:21: error: macro "hard_reset" passed 1 arguments, but takes just 0
 void hard_reset(void)
                     ^
src/soc/intel/common/reset.c:37:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
 {
 ^
make: *** [build/bootblock/soc/intel/common/reset.o] Error 1

TEST=Build and run on Galileo Gen2

Change-Id: I793570e62a0e46cca86cc540c243e363896ceac7
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/15988
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/common/reset.c

@@ -33,6 +33,7 @@
 
 void WEAK reset_prepare(void) { /* do nothing */ }
 
+#if IS_ENABLED(CONFIG_HAVE_HARD_RESET)
 void hard_reset(void)
 {
 	reset_prepare();
@@ -41,6 +42,7 @@ void hard_reset(void)
 	while (1)
 		hlt();
 }
+#endif
 
 void soft_reset(void)
 {