Fixed compilation warnings

test_programs/riscv_instructions/simple_arithmetics/unsigned_addition.c

@@ -1,5 +1,5 @@
 // EXPECTS TWO VARIABLES WITH A VALUE OF UNSIGNED 1
-void main() {
+int main() {
     unsigned int x = 0;
     unsigned int y = 1;
     x = x + y;

test_programs/riscv_instructions/simple_arithmetics/unsigned_division.c

@@ -1,5 +1,5 @@
 // Expecting two variables with a value of two
-void main() {
+int main() {
     unsigned int x = 4;
     unsigned int y = 2;
     x = x / y;

test_programs/riscv_instructions/simple_arithmetics/unsigned_multiplication.c

@@ -1,5 +1,5 @@
 // EXPECTS TWO VARIABLES WITH A VALUE OF UNSIGNED 2
-void main() {
+int main() {
     unsigned int x = 1;
     unsigned int y = 2;
     x = x * y;

test_programs/riscv_instructions/simple_arithmetics/unsigned_substraction.c

@@ -1,5 +1,5 @@
 // EXPECTS TWO VARIABLES WITH A VALUE OF UNSIGNED 1
-void main() {
+int main() {
     unsigned int x = 1;
     unsigned int y = 1;
     x = x - y;