webkitgtk: Fix build break on non-arm/non-x86 systems

webkitgtk: Fix build break on non-arm/non-x86 systems

Fixes
/webkitgtk-2.48.1/Source/WebCore/platform/audio/DenormalDisabler.cpp:94:47:
   ↪ error: expected ';' after default
   |    94 | DenormalDisabler::DenormalDisabler() = default
   |       |                                               ^
   |       |                                               ;

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>