Use <stdarg.h> instead of <machine/stdarg.h> in userland.

<machine/stdarg.h> is a kernel-only header.  The standard header for
userland is <stdarg.h>.  Using the standard header in userland avoids
weird build errors when building with external compilers that include
their own stdarg.h header.

Reviewed by:	arichardson, brooks, imp
Sponsored by:	DARPA / AFRL
Differential Revision:	https://reviews.freebsd.org/D14776

sys/cam/scsi/scsi_all.h

@@ -25,7 +25,11 @@
 #define	_SCSI_SCSI_ALL_H 1
 
 #include <sys/cdefs.h>
+#ifdef _KERNEL
 #include <machine/stdarg.h>
+#else
+#include <stdarg.h>
+#endif
 
 #ifdef _KERNEL
 /*