package/webkitgtk: needs >= GCC 7

CMakeLists.txt contains a toolchain check:

if (${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
    if (${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS "7.3.0")
        message(FATAL_ERROR "GCC 7.3 or newer is required to build WebKit. Use a newer GCC version or Clang.")
    endif ()
endif ()

So bump the toolchain dependency to >= GCC 7.  The check is really about >=
7.3.0, but we do not have such detailed version checks.  Given that GCC
7.3.0 was released in January 2018 (and 7.1.0 in May 2017), most external
GCC 7.x toolchains probably use >= 7.3.0.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>