Make compilation succeed with gnu90

[giulianoriccio]

Fixes the following compilation error (see #121):

src/spx_resource_stats-linux.c: In function 'spx_resource_stats_own_rss': src/spx_resource_stats-linux.c:118:5: error: 'for' loop initial declarations are only allowed in C99 mode for (size_t i = 0; i < sizeof(context.lg_buf); i++) { ^ src/spx_resource_stats-linux.c:118:5: note: use option -std=c99 or -std=gnu99 to compile your code

[NoiseByNorthwest]

Thanks, I'll add the -std=gnu90 flag in order to avoid this kind of issue in the future.