prosody: fix build with musl libc

prosody: fix build with musl libc

musl libc does not provide the mallinfo() API. Define WITHOUT_MALLINFO
to skip the code using it.

Fixes:
http://autobuild.buildroot.net/results/42c/42cd6e3be761d1cb358bc50acd7fa58c8aa1c750/
http://autobuild.buildroot.net/results/29d/29d8dcbe013e549a865c0c5953272dab1fe5f777/
http://autobuild.buildroot.net/results/a6d/a6deb7440f80b072432c0f9c048e59abe4edf1c0/

Cc: Dushara Jayasinghe <nidujay@gmail.com>
Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>