libelf: Make elf_strptr index correctly into compressed section data.

elf_strptr indexes into the section data. This is defined as index into
the uncompressed data of the section. If the section is compressed make
sure the uncompressed data is available, but don't really decompress the
section header (elf_getdata will still return compressed data).

Signed-off-by: Mark Wielaard <mjw@redhat.com>