KEYS: The RSA public key algorithm needs to select MPILIB

The RSA public key algorithm needs to select MPILIB directly in Kconfig as the
'select' directive is not recursive and is thus MPILIB is not enabled by
selecting MPILIB_EXTRA.

Without this, the following errors can occur:

	crypto/built-in.o: In function `RSA_verify_signature':
	rsa.c:(.text+0x1d347): undefined reference to `mpi_get_nbits'
	rsa.c:(.text+0x1d354): undefined reference to `mpi_get_nbits'
	rsa.c:(.text+0x1d36e): undefined reference to `mpi_cmp_ui'
	rsa.c:(.text+0x1d382): undefined reference to `mpi_cmp'
	rsa.c:(.text+0x1d391): undefined reference to `mpi_alloc'
	rsa.c:(.text+0x1d3b0): undefined reference to `mpi_powm'
	rsa.c:(.text+0x1d3c3): undefined reference to `mpi_free'
	rsa.c:(.text+0x1d3d8): undefined reference to `mpi_get_buffer'
	rsa.c:(.text+0x1d4d4): undefined reference to `mpi_free'
	rsa.c:(.text+0x1d503): undefined reference to `mpi_get_nbits'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>

crypto/asymmetric_keys/Kconfig

@@ -23,6 +23,7 @@ config ASYMMETRIC_PUBLIC_KEY_SUBTYPE
 config PUBLIC_KEY_ALGO_RSA
 	tristate "RSA public-key algorithm"
 	select MPILIB_EXTRA
+	select MPILIB
 	help
 	  This option enables support for the RSA algorithm (PKCS#1, RFC3447).