fix(diagnostics): include <poll.h> only on POSIX targets

Process.cpp included <poll.h> unconditionally, above the
!defined(_WIN32) block every other POSIX header already lives in. poll()
is used only on the POSIX path, but the include alone made the file -- and
with it sharp_runtime_diagnostics -- fail to compile for every Windows
target: CNA's MinGW cross-build (CNA_PLATFORM=WIN32) stopped at
"fatal error: poll.h: No such file or directory", and MSVC has no such
header either. The include moves into the POSIX block.

Found by CNA's native-platform validation (libcna/cna
plans/plan_native_platform_validation.md NPV-0027); with it the WIN32,
CNA_ENABLE_SDL=OFF, DIRECTX11;DIRECTX12;SOFTWARE;HEADLESS cross-build of
cna_demo_2d.exe completes.

Author: robertvokac

modules/diagnostics/src/System/Diagnostics/Process.cpp

@@ -4,7 +4,6 @@
 #include "System/Diagnostics/Process.hpp"
 #include <atomic>
 #include <mutex>
-#include <poll.h>
 #include "System/ArgumentException.hpp"
 #include "System/ArgumentOutOfRangeException.hpp"
 #include "System/InvalidOperationException.hpp"
@@ -16,6 +15,7 @@
 #include <sstream>
 
 #if !defined(_WIN32) && !defined(__EMSCRIPTEN__)
+#  include <poll.h>
 #  include <sys/wait.h>
 #  include <sys/types.h>
 #  include <unistd.h>