package/valgrind: disable for static-only builds

Fixes:
http://autobuild.buildroot.net/results/3b6/3b6f253f30afaee428ea0fc559be210dcec61dae
http://autobuild.buildroot.net/results/60e/60e5a49360e2ee9482b4d548789f874a6502c429
http://autobuild.buildroot.net/results/5cc/5ccc2e48fa4deb90ab214c82e8921ec494555457
http://autobuild.buildroot.net/results/452/4524184b407bb6f9b55f8f85c8e0b475ff105420



Valgrind ordinarily works by preloading a shared object to replace malloc,
free and friends. Some combinations of toolchain/libc allow this to build
with -static, but others complain.

Since it makes little sense to try to build this on a static-only system,
this patch disables it on static platforms entirely.

[Peter: move comment to top of file to fix menuconfig indentation]
Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>