Mark all symbols hidden by default, export only required ones

src/util/rw_blocking.h

@@ -6,8 +6,12 @@
 #include <unistd.h>
 #include <sys/uio.h>
 
+#pragma GCC visibility push(default)
+
 size_t read_blocking(int fd, void *read_buf, size_t remaining);
 size_t write_blocking(int fd, void *write_buf, size_t remaining);
 int readv_blocking(int fd, iovec *iov, int iovcnt);
 int writev_blocking(int fd, iovec *iov, int iovcnt);
 int sendv_blocking(int fd, iovec *iov, int iovcnt, int flags);
+
+#pragma GCC visibility pop