libsdl2: Disable libunwind dependency in native builds

libsdl2: Disable libunwind dependency in native builds

libunwind-native currently cannot be added to the class-native DEPENDS
due to the following problem:

https://autobuilder.yoctoproject.org/typhoon/#/builders/91/builds/5624/steps/12/logs/stdio

And without libunwind-native, libsdl2-native config fails errors like:

| -- Checking for one of the modules 'libunwind'
| CMake Error at [...]/build/tmp/work/x86_64-linux/libsdl2-native/2.0.20-r0/recipe-sysroot-native/usr/share/cmake-3.22/Modules/FindPkgConfig.cmake:890 (message):
|   None of the required 'libunwind' found
| Call Stack (most recent call first):
|   CMakeLists.txt:1367 (pkg_search_module)

Ideally, the libunwind issue would be fixed, and then this could be added
to the libsdl2 recipe:

DEPENDS:append:class-native = " libunwind-native"

However, the libunwind problem is not trivial to fix, and requires a
specific setup to reproduce. As an alternative workaround, turn off
libunwind support in the native SDL2 build by disabling the unwind.h
check in the CMake script (this subsequently turns off all the other
libunwind checks). The main user of libsdl2-native is qemu-native, which
does not need libunwind support in SDL2.

Signed-off-by: Carlos Rafael Giani <crg7475@mailbox.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>