stella: fix error when building with uClibc-ng

stella: fix error when building with uClibc-ng

The uClibc-ng C library defines a type called R0 that conflits with a
global variable called R0 from PaddleReader.cxx.

src/emucore/tia/PaddleReader.cxx:25:3: error: ‘constexpr const double R0’ redeclared as different kind of symbol
   R0 = 1.5e3,
   ^

/home/sprado/workspace/build/buildroot/build/stella_uclibc/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/sys/ucontext.h:42:3:note: previous declaration ‘<anonymous enum> R0’
   R0 = 0,
   ^~

So let's redefine R0 as a private member of the PaddleReader class.

This patch has already been accepted upstream [1] and should be removed in
the next version bump.

[1] https://github.com/stella-emu/stella/commit/391601d2386e41372d6fa198fbe40287d2e87542

Fixes:

http://autobuild.buildroot.net/results/9ab5772f01236eaa7def66a2f443f13efca8c34c
http://autobuild.buildroot.net/results/5a4a194fe85e04ab31b146367c5a45e4f7688fca
http://autobuild.buildroot.net/results/31f6bb3f5879d056fe152ca19b6b0367cc636212
...

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>