internal/rosa/zlib: compile with -fPIC

For static linking into shared libraries. This was missed when migrating to CMake.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/zlib.go

@@ -15,6 +15,7 @@ func (t Toolchain) newZlib() (pkg.Artifact, string) {
 		Cache: []KV{
 			{"CMAKE_BUILD_TYPE", "Release"},
 
+			{"CMAKE_C_FLAGS", "-fPIC"},
 			{"ZLIB_BUILD_TESTING", "OFF"},
 			{"ZLIB_BUILD_SHARED", "ON"},
 			{"ZLIB_BUILD_STATIC", "ON"},