TSCBasic: FILE is an opaque struct since Android 7

[finagolfin]

I've been working around this for awhile by passing in -Xcc -D__ANDROID_API__=23 when cross-compiling SPM for Android with this file. Otherwise, I would get these errors:

/home/butta/src/swift-tools-support-core/Sources/TSCBasic/WritableByteStream.swift:685:51: error: cannot find type 'FILE' in scope
    public init(filePointer: UnsafeMutablePointer<FILE>, closeOnDeinit: Bool = true, buffered: Bool = true) throws {
/home/butta/src/swift-tools-support-core/Sources/TSCBasic/Process.swift:546:70: error: cannot convert value of type 'OpaquePointer?' to expected argument type 'AbsolutePath'
        let stdinStream = try LocalFileOutputByteStream(filePointer: fdopen(stdinPipe[1], "wb"), closeOnDeinit: true)

This is because a definition was added for Android 6, ie API 23, and below, but it's an opaque struct for all recent Android APIs, aosp-mirror/platform_bionic@3037ea4. This pull gets this Swift file to compile without having to force the Android API to 23 with the above flags.

[neonichu]

@swift-ci please test

[finagolfin]

Ping, ready to go?