speechPlayer: fix sample type qualification to avoid ambiguity (#1722)

As reported in <https://bugs.freebsd.org/271486>, when the C++ standard
library provides std::sample, it causes a compile error when compiling
`speechWaveGenerator.cpp`:

```
  src/speechPlayer/src/speechWaveGenerator.cpp:197:56: error: reference to 'sample' is ambiguous
          unsigned int generate(const unsigned int sampleCount, sample* sampleBuf) {
                                                                ^
  src/speechPlayer/src/sample.h:23:3: note: candidate found by name lookup is 'sample'
  } sample;
    ^
  /usr/include/c++/v1/__algorithm/sample.h:95:17: note: candidate found by name lookup is 'std::sample'
  _SampleIterator sample(_PopulationIterator __first,
                  ^
```

Since `speechWaveGenerator.cpp` puts `using namespace std;` at the top,
fix the error by qualifying `sample` with an explicit `::`.

src/speechPlayer/src/speechWaveGenerator.cpp

@@ -194,7 +194,7 @@ class SpeechWaveGeneratorImpl: public SpeechWaveGenerator {
 	SpeechWaveGeneratorImpl(int sr): sampleRate(sr), voiceGenerator(sr), fricGenerator(), cascade(sr), parallel(sr), frameManager(NULL) {
 	}
 
-	unsigned int generate(const unsigned int sampleCount, sample* sampleBuf) {
+	unsigned int generate(const unsigned int sampleCount, ::sample* sampleBuf) {
 		if(!frameManager) return 0; 
 		for(unsigned int i=0;i<sampleCount;++i) {
 			const speechPlayer_frame_t* frame=frameManager->getCurrentFrame();