apk: build static

apk: build static

libapk as a library is used nowhere else and makes no sense to link it
shared. By linking static, we can take advantage of LTO and get the size
reduced further.

Some numbers locally:

Before:

168K staging_dir/target-mips_24kc_musl/root-ath79/usr/bin/apk*
252K staging_dir/target-mips_24kc_musl/root-ath79/usr/lib/libapk.so.2.99.0*

Static:
344K staging_dir/target-mips_24kc_musl/root-ath79/usr/bin/apk*

Static + LTO:
305K staging_dir/target-mips_24kc_musl/root-ath79/usr/bin/apk*

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17109
Signed-off-by: Robert Marko <robimarko@gmail.com>