hurd: Fix mach installed headers test

	* include/lock-intern.h [!_ISOMAC]: Do not declare libc hidden
	prototypes.
	* include/mach.h [!_ISOMAC]: Likewise.
	* include/mach/mig_support.h [!_ISOMAC]: Likewise.
	* include/mach_error.h [!_ISOMAC]: Likewise.