squid: disable krb5

- fix build error reported by buildbot by disabling krb5
- libcom_err from krb5 is used - configure output:
configure: WARNING: library 'com_err' is required for Heimdal Kerberos
- krb5 has its own libcom_err (see krb5 package) with its own symbols

- linking with wrong libcom_err from libext2fs produces errors during
libgssapi_krb5.so: undefined reference to `error_message@com_err_3_MIT'
libgssapi_krb5.so: undefined reference to `remove_error_table@com_err_3_MIT'
libgssapi_krb5.so: undefined reference to `add_error_table@com_err_3_MIT'
-> remove libext2fs dependency (wasnt working anyway - no PKG_BUILD_DEP)

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>