fixed: mark cblas with C linkage

this does not change anything, as the symbols always had C linkage.
but not all cblas.h versions includes the extern "C"

git-svn-id: http://svn.sintef.no/trondheim/IFEM/trunk@1864 e10b68d5-8a6e-419e-a041-bce267b0401d

src/LinAlg/matrix.h

@@ -30,7 +30,10 @@
 #include <mkl_cblas.h>
 #pragma pop_macro("real")
 #elif defined(USE_CBLAS)
+extern "C"
+{
 #include <cblas.h>
+}
 #endif
 
 #ifdef INDEX_CHECK