pkg-autotools: fix whitespace regex in libtool patch hook

The first sed line for ltmain_version attempts to match whitespace before
VERSION with [ space_character tab_character ]. The second sed line used to
make ltmain_patchlevel has only space characters in the whitespace expression,
possibly due to a smart editor automatically converting tabs to spaces. Make
these lines consistent with each other and more resilient against accidental
loss of tab characters by replacing the tabs with '\t'.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>