Use a single WS_NORETURN macro

Use a single WS_NORETURN macro

Having to define two macros for marking a function as never returning
seems a bit redundant. Merge the MSVC and GCC-like attributes into a
single WS_NORETURN.

Tested with Clang 3.7.1, GCC 4.4.7 and even GCC 4.1.2 using this small
program (-Wall -Wextra, the first two generate warnings for
uninitialized variables, the last one compiles without warnings):

    #include <stdlib.h>
    __attribute__((noreturn)) void foo() { exit(1); }
    __attribute__((noreturn)) void bar();
    void bar() { exit(1); }
    int main() {
      int j, i;
      if (i) { bar(); return j; }
      foo();
      return j;
    }

Change-Id: I7d19c15e61b8f8fa4936864407199c4109f8cc82
Reviewed-on: https://code.wireshark.org/review/14822
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>