Fix bug #107: Turn Windows path to Linux one

[hbf17517]

The value of $ANDROID_NDK_HOME in my environment is C:\Users\rob\AppData\Local\Android\Sdk\ndk\28.0.13004108

ndk_path is set to that value, while p in line 112 is set to the paths using forward slashes, leading to failure of string.replace to find the substring ndk_path. See https://github.com/bazelbuild/rules_android_ndk/issues/107

Solution: Reset ndk_path to use forward slashes.

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