Only build ipmi_linux on x86

Somehow this wasn't triggered in my pre-commit build.

sys/modules/ipmi/Makefile

@@ -1,7 +1,5 @@
 # $FreeBSD$
 
-SUBDIR+= ipmi_linux
-
 .PATH: ${SRCTOP}/sys/dev/ipmi
 
 # XXX - ipmi_smbus and ipmi_ssif depend on smbus
@@ -14,6 +12,9 @@ SRCS+=	ipmi_kcs.c ipmi_smic.c ipmi_smbios.c ipmi_ssif.c
 SRCS+=	ipmi_acpi.c ipmi_isa.c ipmi_pci.c ipmi_smbus.c
 SRCS+=	opt_acpi.h
 SRCS+=	acpi_if.h isa_if.h pci_if.h smbus_if.h
+
+SUBDIR+= ipmi_linux
+
 .elif ${MACHINE_ARCH} == "powerpc64"
 SRCS+=	ipmi_opal.c
 .endif