scripts: snippets: Fix path output on windows

Fixes an issue with paths being output in windows-style with back
slashes, this causes issues for certain escape sequences when
cmake interprets them. Replace these paths with posix paths so
that they are not treated as possible escape sequences.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

scripts/snippets.py

@@ -56,7 +56,7 @@ class Snippet:
                 path = pathobj.parent / value
                 if not path.is_file():
                     _err(f'snippet file {pathobj}: {variable}: file not found: {path}')
-                return f'"{path}"'
+                return f'"{path.as_posix()}"'
             if variable in ('DTS_EXTRA_CPPFLAGS'):
                 return f'"{value}"'
             _err(f'unknown append variable: {variable}')