fossil: Explicitly link with libm

fossil: Explicitly link with libm

Fixes the following build error with an external toolchain:

./bld/sqlite3.o: In function `fts5Bm25Function':
sqlite3.c:(.text+0x27234): undefined reference to `log'
./bld/piechart.o: In function `piechart_render':
piechart_.c:(.text+0x430): undefined reference to `sincos'
piechart_.c:(.text+0x47c): undefined reference to `sincos'
piechart_.c:(.text+0x548): undefined reference to `sincos'
collect2: error: ld returned 1 exit status
src/main.mk:526: recipe for target 'fossil' failed
make[3]: *** [fossil] Error 1

Signed-off-by: Florian Fainelli <redacted>