Add feature to change currency to GBP

src/currencyExchange.c

@@ -5,4 +5,11 @@ float toUSD(float euro) {
     float USD = euro * CURRENT_USD_RATE_OF_ONE_EURO;
 
     return USD;
+}
+
+float toGBP(float euro) {
+
+    float GBP = euro * CURRENT_GBP_RATE_OF_ONE_EURO;
+
+    return GBP;
 }