tests/lib/coreboot_table-test.c: Add lb_string_platform_blob_version

A later patch changes the .config file which we use for our tests.
However that causes the PLATFORM_USES_FSP2_0 option to be enabled, which
in turn causes build errors in our tests, because the function is
obviously not defined in our tests. Create a stub function of sorts like
we do for other coreboot table entries.

It also moves the declaration of the `lb_string_platform_blob_version`
function to coreboot_tables.h, since it doesn't belong in the FSP header
file. Because of that we can also remove the
`if (CONFIG_PLATFORM_USES_FSP2_0)`
check, which makes the code a bit cleaner.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I7721dfe4d287b2274a383bb7e5337b85a0f3f148
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90830
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>

src/include/boot/coreboot_tables.h

@@ -24,6 +24,8 @@ void lb_add_console(uint16_t consoletype, void *data);
 
 enum cb_err fill_lb_pcie(struct lb_pcie *pcie);
 
+void lb_string_platform_blob_version(struct lb_header *header);
+
 /* Define this in mainboard.c to add board-specific table entries. */
 void lb_board(struct lb_header *header);