string/endian.h: Restore the __USE_MISC conditionals

Commit 69fd157a "time: Add padding for the timespec if required"
caused a breakage in the glibc tests as the endian.h include file was
kept in the networking headers while the __USE_MISC #ifdefs had been
removed. This resulted in namespace violations in the networking
headers.

This patche restores the __USE_MISC conditionals in endian.h to fix the
test failures.

	* string/endian.h: Restore the __USE_MISC conditionals.