forward: also include paths.h

It provides several important constants, especially _PATH_BSHELL, which
is used in PID1, executor, and run. The header has been included
indirectly through e.g. libmount.h, mntent.h, utmpx.h, and so on.
Let's explicitly include it in forward.h, as libmount.h and friends that
includes paths.h are irrelevant to _PATH_BSHELL, and we may easily fail
to build when code is touched.
The header is not heavy, hence should not hurt anything.

src/basic/forward.h

@@ -6,6 +6,7 @@
 #include <errno.h>              /* IWYU pragma: export */
 #include <inttypes.h>           /* IWYU pragma: export */
 #include <limits.h>             /* IWYU pragma: export */
+#include <paths.h>              /* IWYU pragma: export */
 #include <stdarg.h>             /* IWYU pragma: export */
 #include <stdbool.h>            /* IWYU pragma: export */
 #include <stddef.h>             /* IWYU pragma: export */