math/tmv: fix build on powerpc

c++ -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -isystem /usr/local/include -I../include -fopenmp -fPIC -DTMV_NDEBUG -DFBLAS -DNOLAP -isystem /usr/local/include -c -o TMV_MultMM_OpenMP.o TMV_MultMM_OpenMP.cpp
TMV_MultMM.cpp:33:10: fatal error: 'omp.h' file not found