Add public API to seed PRNG

src/include/espeak-ng/espeak_ng.h

 ESPEAK_NG_API espeak_ng_STATUS
 espeak_ng_SetConstF0(int f0);
 
+ESPEAK_NG_API espeak_ng_STATUS
+espeak_ng_SetRandSeed(long seed);
+
 
 #ifdef __cplusplus
 }

src/libespeak-ng/common.c

 	(void)espeak_rand(0, 1); // Dummy flush a generator
 }
 
+#pragma GCC visibility push(default)
+ESPEAK_NG_API espeak_ng_STATUS
+espeak_ng_SetRandSeed(long seed) {
+	espeak_srand(seed);
+	return ENS_OK;
+}
+#pragma GCC visibility pop