Fix compile error in cpp tutorial (#380)

Not tested but looks legitimate + better than previous state in any case ;-)

tutorial/cpp/2-IVFFlat.cpp

@@ -11,7 +11,7 @@
 #include <cassert>
 
 #include <faiss/IndexFlat.h>
-#include <faiss/IndexIVF.h>
+#include <faiss/IndexIVFFlat.h>
 
 
 int main() {

tutorial/cpp/Makefile

@@ -14,13 +14,13 @@ cpu: 1-Flat 2-IVFFlat 3-IVFPQ
 gpu: 4-GPU 5-Multiple-GPUs
 
 1-Flat: 1-Flat.cpp ../../libfaiss.$(SHAREDEXT)
-	$(CC) -o $@ $(CFLAGS) $< -I../../../ $(LDFLAGS)
+	$(CXX) -o $@ $(CXXFLAGS) $< -I../../../ $(LDFLAGS)
 
 2-IVFFlat: 2-IVFFlat.cpp ../../libfaiss.$(SHAREDEXT)
-	$(CC) -o $@ $(CFLAGS) $< -I../../../ $(LDFLAGS)
+	$(CXX) -o $@ $(CXXFLAGS) $< -I../../../ $(LDFLAGS)
 
 3-IVFPQ: 3-IVFPQ.cpp ../../libfaiss.$(SHAREDEXT)
-	$(CC) -o $@ $(CFLAGS) $< -I../../../ $(LDFLAGS)
+	$(CXX) -o $@ $(CXXFLAGS) $< -I../../../ $(LDFLAGS)
 
 4-GPU: 4-GPU.cpp ../../libfaiss.$(SHAREDEXT) ../../gpu/libgpufaiss.$(SHAREDEXT)
 	$(NVCC) $(NVCCFLAGS) -o $@ $< $(NVCCLDFLAGS) -I../../../