FAST_DEPEND: Fix improperly depending all .So objects on all headers.

This was a regression in r290629, which was revealed partly in r294360.
Once 'make depend' has ran it will generate all headers already.  Thus
even with FAST_DEPEND lacking proper dependencies before building, it
will not have any missing headers.  Once objects are compiled the depend
files will be generated with proper dependencies.

Sponsored by:	EMC / Isilon Storage Division

share/mk/bsd.lib.mk

@@ -433,7 +433,9 @@ ${_S:R}.po: ${_S}
 .endif
 .if defined(SHLIB_NAME) || \
     defined(INSTALL_PIC_ARCHIVE) && defined(LIB) && !empty(LIB)
+.if !exists(${.OBJDIR}/${DEPENDFILE})
 ${SOBJS}: ${SRCS:M*.h}
+.endif
 .for _S in ${SRCS:N*.[hly]}
 ${_S:R}.So: ${_S}
 .endfor