alsamixer: fix build on uClibc

exp10 is a glibc extension, which isn't supported on uClibc. Luckily,
exp10() is trivial to compute based on exp(), so add a wrapper for
the uClibc case.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>