fix include path

bench/bench_gemm.cpp

diff --git a/bench/bench_gemm.cpp b/bench/bench_gemm.cpp
index 98ac34e..41ca8b3 100644
--- a/bench/bench_gemm.cpp
+++ b/bench/bench_gemm.cpp
@@ -24,7 +24,7 @@
 #ifdef HAVE_BLAS
 
 extern "C" {
-  #include <bench/btl/libs/C_BLAS/blas.h>
+  #include <Eigen/src/misc/blas.h>
 }
 
 static float fone = 1;