Allow context types with an ALT version to be linked in Doxygen

In our tests, we run `tests/scripts/doxygen.sh`, which checks that `doxygen`
runs without warnings after `scripts/config.py realfull`. In this
configuration, alternative implementations such as `MBEDTLS_RSA_ALT` are
enabled, which allows the documentation to contain references to the
`MBEDTLS_xxx_ALT` symbol itself. However, this disables context types that
alternative implementations must define in their header, such as
`mbedtls_rsa_context`. See https://github.com/Mbed-TLS/mbedtls/issues/4518

As a partial fix, allow `tests/scripts/doxygen.sh` to see dummy definitions
of the context type. This way, we can use both `#MBEDTLS_RSA_ALT` and
`#mbedtls_rsa_context` cross-references in our documentation. This is not
ideal, because `doxygen.sh` isn't testing for errors in the documentation of
the affected context types, but it's cheap progress.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

doxygen/mbedtls.doxyfile

@@ -6,7 +6,7 @@ EXTRACT_ALL            = YES
 EXTRACT_PRIVATE        = YES
 EXTRACT_STATIC         = YES
 CASE_SENSE_NAMES       = NO
-INPUT                  = ../include input
+INPUT                  = ../include input ../tests/include/alt-dummy
 FILE_PATTERNS          = *.h
 RECURSIVE              = YES
 EXCLUDE_SYMLINKS       = YES