Make some buffers non-static

These don't actually need to survive function exit.

src/libespeak-ng/numbers.c

@@ -479,7 +479,7 @@ void LookupLetter(Translator *tr, unsigned int letter, int next_byte, char *ph_b
 	// control, bit 0:  not the first letter of a word
 
 	int len;
-	static char single_letter[10] = { 0, 0 };
+	char single_letter[10] = { 0, 0 };
 	unsigned int dict_flags[2];
 	char ph_buf3[40];
 

src/libespeak-ng/voices.c

@@ -1519,7 +1519,7 @@ ESPEAK_NG_API espeak_ng_STATUS espeak_ng_SetVoiceByFile(const char *filename)
 	int ix;
 	espeak_VOICE voice_selector;
 	char *variant_name;
-	static char buf[60];
+	char buf[60];
 
 	strncpy0(buf, filename, sizeof(buf));
 
@@ -1556,7 +1556,7 @@ ESPEAK_NG_API espeak_ng_STATUS espeak_ng_SetVoiceByName(const char *name)
 	int ix;
 	espeak_VOICE voice_selector;
 	char *variant_name;
-	static char buf[60];
+	char buf[60];
 
 	strncpy0(buf, name, sizeof(buf));