correcting makefile dependencies

Makefile

@@ -3,7 +3,7 @@ CXXFLAGS = -O0 -g -std=c++17 -Wall -Wextra -Wpedantic
 a.out: exec.o  Graph.o Edge.o Node.o
 	$(CXX) $(CXXFLAGS) -o $@ $^
 
-exec.o: exec.cpp alg.o Graph.o
+exec.o: exec.cpp alg.cpp Graph.o
 	$(CXX) $(CXXFLAGS) -c $<
 
 Graph.o: Graph.cpp latex.cpp Graph.h Edge.o Node.o preprocessor_compile_control.h