subr_physmem: Fix userspace build

Include stdbool.h in userspace configurations. For the kernel builds we
get it from sys/types.h, but bool isn't defined there for non-kernel
builds and this otherwise kernel-only file is used for the physmem test
suite.

Fixes:			deb1e3b719
Sponsored by:		Netflix

sys/sys/physmem.h

@@ -31,6 +31,10 @@
 #ifndef	_SYS_PHYSMEM_H_
 #define	_SYS_PHYSMEM_H_
 
+#ifndef _KERNEL
+#include <stdbool.h>
+#endif
+
 /*
  * Routines to help configure physical ram.
  *