Fix C.UTF-8 locale definition

[Xi Ruoyao]

In Glibc we have:

localedef -i POSIX -f UTF-8 C.UTF-8 2> /dev/null || true

This is causing a test failure in epiphany-45.2:

not ok /src/ephy-encodings/create - epiphany:ERROR:../tests/ephy-encodings-test.c:47:test_ephy_encodings_create: assertion failed (ephy_encoding_get_collation_key (encoding) == "Unicode (UTF-8)"): ("Wpkeqfg\"*WVH/:+" == "Unicode (UTF-8)")
Bail out!

To me we should just use

localedef -i C -f UTF-8 C.UTF-8

instead. This also removes the necessity of 2>/dev/null || true.