contrib: update gnutls to 3.7.6

contrib/src/gnutls/0001-m4-remove-malloc-realloc.patch

contrib/src/gnutls/SHA512SUMS

contrib/src/gnutls/downgrade-gettext-requirement.patch

contrib/src/gnutls/gnutls-3.6.7-win32-compat.patch

contrib/src/gnutls/gnutls-3.6.7-win32-vs-support.patch

contrib/src/gnutls/gnutls-no-egd.patch

contrib/src/gnutls/gnutls-uwp.patch

contrib/src/gnutls/gnutls_tasn.patch

contrib/src/gnutls/no-create-time-h.patch

contrib/src/gnutls/read-file-limits.h.patch

contrib/src/gnutls/rules.mak