Makefile.ps5: Add missing assets to list.

[rootlis]

install-ps5.c includes 6 assets, but only 3 are listed in the Makefile. Missing ftpsrv-ps5.elf from the list allows make to try building ftpsrv-ps5-install.elf before ftpsrv-ps5.elf is available.

That can happen, for example, when a build script automatically sets the environment variable MAKEFLAGS="-j2", resulting in:

/opt/ps5-payload-sdk/bin/prospero-clang -Wall -Werror -DVERSION_TAG=\"d5cf56-dirty\" -I. -DTITLE_ID=\"BREW10001\" -o ftpsrv-ps5.elf srv.c cmd.c notify.c io.c self.c sha256.c main-prospero.c cmd-prospero.c self-prospero.c io-freebsd.c
/opt/ps5-payload-sdk/bin/prospero-clang -Wall -Werror -DVERSION_TAG=\"d5cf56-dirty\" -I. -DTITLE_ID=\"BREW10001\" -o ftpsrv-ps5-install.elf install-ps5.c -lSceIpmi -lSceAppInstUtil
<inline asm>:7:9: error: Could not find incbin file 'ftpsrv-ps5.elf'
    7 | .incbin "ftpsrv-ps5.elf"
      |         ^
1 error generated.
make: *** [Makefile:43: ftpsrv-ps5-install.elf] Error 1
make: *** Waiting for unfinished jobs....

This PR fixes the build failure bug and includes the other 2 missing assets for completeness.

[john-tornblom]

Thanks! :)