Add procfs_find_LDT_entry declaration (procfs.h).

Add procfs_find_LDT_entry declaration (procfs.h).

Add a declaration for procfs_find_LDT_entry in order to prevent
a compiler warning about this function missing one. This also
helps making sure that declaration and definition remain consistent.

gdb/ChangeLog:

        * procfs.h (procfs_find_LDT_entry): Add declaration.
        * sol-thread.c (ps_lgetLDT): Delete local declaration of
        function procfs_find_LDT_entry.