bug(core): Getting a warning when linking statically with libkeymancore

On Linux, running the script below which builds an app with static linking results in a warning:

```
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/libicuuc.a(putil.ao): in function `uprv_dl_open_70':
(.text+0x1936): warning: Using 'dlopen' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
```

We should fix the warning (which probably requires static linking to ICU for the static libkeymancore library), or not ship the static version of libkeymancore (and remove the dependency on `libicu-dev` from `linux/debian/control` and the `requires.private` line from the `keyman_core.pc` file).

```bash
cat < keymantest.c
#include

int main(int argc, char *argv[]) {
km_core_option_item opts[] = {KM_CORE_OPTIONS_END};
km_core_keyboard *kb = NULL;
km_core_state *state = NULL;
km_core_keyboard_load(NULL, &kb);
km_core_state_create(kb, opts, &state);
km_core_actions const *a = km_core_state_get_actions(state);
}
EOF

# shellcheck disable=SC2046 disable=SC2312
g++ keymantest.c -static $(pkg-config --cflags --libs keyman_core) \
$(pkg-config --cflags --libs icu-uc) $(pkg-config --cflags --libs icu-i18n)
```

This should also be added as an autopkgtest to `linux/debian/tests/test-build`.