build: use Requires.private for OpenSSL in static pkg-config file

Change Requires to Requires.private in the static library's .pc file.

Requires.private means pkg-config only exposes the dependency's flags
when the consumer passes --static. Without --static, the dependency
is ignored entirely. This is the correct semantics because:

- OpenSSL is an internal implementation detail, not exposed in the
  public API headers. Consumers don't need OpenSSL's -I flags to
  compile against the driver.
- Anyone linking the static .a should be using --static, which pulls
  in Requires.private deps. So static consumers still get -lssl.
- If a consumer mistakenly uses the static .pc without --static
  (e.g. linking the .a directly), Requires would needlessly add
  OpenSSL flags that may conflict with their own OpenSSL usage.

This matches the convention used by OpenSSL's own libssl.pc, which
declares Requires.private: libcrypto rather than Requires:.

Author: wprzytula

scylla-rust-wrapper/scylla-cpp-driver_static.pc.in

@@ -6,7 +6,7 @@ includedir=@includedir@
 Name: scylla-cpp-rs-driver
 Description: ScyllaDB CPP RS Driver
 Version: @version@
-Requires: openssl
+Requires.private: openssl
 Libs: -L${libdir} -lscylla-cpp-driver_static
 Cflags: -I${includedir}
 URL: https://github.com/scylladb/cpp-rs-driver/