Fix gcc sign conversion warnings in compilembrola.c.

src/libespeak-ng/compilembrola.c

 {
 	// Pack 4 characters into a word
 	int ix;
-	unsigned char c;
 	unsigned int word;
 
 	if (string == NULL)
 	word = 0;
 	for (ix = 0; ix < 4; ix++) {
 		if (string[ix] == 0) break;
-		c = string[ix];
-		word |= (c << (ix*8));
+		word |= ((unsigned int)string[ix] << (ix*8));
 	}
 	return word;
 }

src/libespeak-ng/synthesize.h

 } SOUND_ICON;
 
 typedef struct {
-	int name;
+	unsigned int name;
 	unsigned int next_phoneme;
-	int mbr_name;
-	int mbr_name2;
+	unsigned int mbr_name;
+	unsigned int mbr_name2;
 	int percent; // percentage length of first component
 	int control;
 } MBROLA_TAB;