math/lrslib: fix build on GCC architectures
Use C11 compiler:
lrslong.h:191: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'lrs_mp'
lrslong.h:192: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
lrslong.h:193: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
lrslong.h:194: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
lrslong.h:218: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'lrs_alloc_mp_t'
lrslong.h:219: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'lrs_alloc_mp_vector'