arch/x86/include/arch/mmio.h: Provide __always_inline definition for musl

fix compilation on musl-libc systems by providing an implementation
for __always_inline

Signed-off-by: Fabian Groffen <grobian@gentoo.org>
Change-Id: I01a7eb9ed28e79523623ab362510ec2d93f4a8b7
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73667
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/include/arch/mmio.h

@@ -5,6 +5,10 @@
 
 #include <stdint.h>
 
+#ifndef __always_inline
+#define __always_inline inline
+#endif
+
 static __always_inline uint8_t read8(const volatile void *addr)
 {
 	return *((volatile uint8_t *)(addr));