library_manager: add missing "depends on CONFIG_LLEXT"

Block the possibility of compiling library_manager without CONFIG_LLEXT,
which obviously fails with dozens of linker errors.

PS: I have no idea why library_manager depends on IPC_MAJOR_4.  Kconfig
is designed to express actual code dependencies, not "supported"
configurations.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

src/library_manager/Kconfig

@@ -7,6 +7,7 @@ menu "Library Manager"
 config LIBRARY_MANAGER
 	bool "Library Manager Support"
 	default n
+	depends on CONFIG_LLEXT
 	depends on IPC_MAJOR_4
 	help
 	  This is support for dynamic modules loading.