api: don't use intermediate variables for deprecation warnings

Using an intermediate constant variable breaks compilation with C
compilers, as these variables cannot be used as initializers.

Instead, define a deprecated type and cast all deprecated symbols to
that type. We lose the information about what the new replacement
symbol is, but we don't break compilation.

E.g.:
    test.c: In function ‘main’:
    test.c:8:5: warning: ‘MMModemBandDeprecated’ is deprecated [-Wdeprecated-declarations]
         printf ("band: %d\n", MM_MODEM_BAND_U2100);
         ^~~~~~

docs/reference/api/ModemManager-sections.txt

@@ -106,4 +106,6 @@ MM_MODEM_BAND_EUTRAN_XLI
 MM_MODEM_BAND_EUTRAN_XLII
 MM_MODEM_BAND_EUTRAN_XLIII
 MM_MODEM_BAND_EUTRAN_XLIV
+<SUBSECTION Private>
+MMModemBandDeprecated
 </SECTION>