FindPortMidi: Link ALSA in static builds on Linux

[fwcd]

Another follow-up to #12291, the never ending story of linker errors in getting Mixxx to compile statically on Linux. PortMidi was apparently missing an ALSA link dependency too, which resulted in similar errors (see this build):

/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libportmidi.a(pmlinuxalsa.c.o): in function `alsa_poll':
pmlinuxalsa.c:(.text+0x9d): undefined reference to `snd_seq_event_input_pending'
/usr/bin/ld: pmlinuxalsa.c:(.text+0xba): undefined reference to `snd_seq_event_input_pending'
/usr/bin/ld: pmlinuxalsa.c:(.text+0xcd): undefined reference to `snd_seq_event_input'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libportmidi.a(pmlinuxalsa.c.o): in function `alsa_write_byte':
pmlinuxalsa.c:(.text+0x510): undefined reference to `snd_midi_event_encode_byte'
/usr/bin/ld: pmlinuxalsa.c:(.text+0x5c5): undefined reference to `snd_seq_event_output'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libportmidi.a(pmlinuxalsa.c.o): in function `alsa_create_virtual':
pmlinuxalsa.c:(.text+0x6c5): undefined reference to `snd_seq_port_info_sizeof'
/usr/bin/ld: pmlinuxalsa.c:(.text+0x70d): undefined reference to `snd_seq_port_info_sizeof'
/usr/bin/ld: pmlinuxalsa.c:(.text+0x730): undefined reference to `snd_seq_port_info_set_capability'
/usr/bin/ld: pmlinuxalsa.c:(.text+0x73d): undefined reference to `snd_seq_port_info_set_type'
...

[daschuer]

Still all green, LGTM.