package/libpfm4: disable Werror

Set `DBG` to an empty value to disable -Werror when building libpfm4. Build
aborts with a musl toolchain because of warnings about redirecting incorrect
header includes.

So -Werror shouldn't be used in released code since it can cause random build
failures on moderate warnings. It also depends on the used toolchain since
different toolchains may or may not print the same warnings.

Fixes:
http://autobuild.buildroot.net/results/6df/6df9b94a79be1dc5ba878f7b67bf9ad4ce2f2e98/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>