code cleanup: static phoneme_callback

src/libespeak-ng/speech.c

 static espeak_ng_STATUS err = ENS_OK;
 
 static t_espeak_callback *synth_callback = NULL;
-int (*phoneme_callback)(const char *) = NULL;
 
 char path_home[N_PATH_HOME]; // this is the espeak-ng-data directory
 extern int saved_parameters[N_SPEECH_PARAM]; // Parameters saved on synthesis start
 #endif
 }
 
-ESPEAK_API void espeak_SetPhonemeCallback(int (*PhonemeCallback)(const char *))
-{
-	phoneme_callback = PhonemeCallback;
-}
-
 ESPEAK_NG_API espeak_ng_STATUS
 espeak_ng_Synthesize(const void *text, size_t size,
                      unsigned int position,

src/libespeak-ng/synthesize.c

 
 static voice_t *new_voice = NULL;
 
+static int (*phoneme_callback)(const char *) = NULL;
+
 #define RMS_GLOTTAL1 35   // vowel before glottal stop
 #define RMS_START 28  // 28
 #define VOWEL_FRONT_LENGTH  50
 
 	return 1;
 }
+
+#pragma GCC visibility push(default)
+ESPEAK_API void espeak_SetPhonemeCallback(int (*PhonemeCallback)(const char *))
+{
+	phoneme_callback = PhonemeCallback;
+}
+#pragma GCC visibility pop

src/libespeak-ng/translate.h

 extern espeak_ng_TEXT_DECODER *p_decoder;
 extern int dictionary_skipwords;
 
-extern int (*phoneme_callback)(const char *);
-
 #define LEADING_2_BITS 0xC0 // 0b11000000
 #define UTF8_TAIL_BITS 0x80 // 0b10000000