guile: fix installed-vs-shipped error

guile: fix installed-vs-shipped error

Fixed:
guile-2.0.11: guile: Files/directories were installed but not shipped
  /usr/lib64/libguile-2.0*-gdb.scm [installed-vs-shipped]

This is because when there is no file in the directory:
for f in libguile-2.0*; do
    [snip]
done

The f would be libguile-2.0* itself, use install-exec-hook will fix the
problem since it depends on install-libLTLIBRARIES.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>