pbio/main: Fix return type of pbio_main_start_application_resources.

[CrispStrobe]

lib/pbio/include/pbio/main.h declares

pbio_error_t pbio_main_start_application_resources(void);

but lib/pbio/src/main.c defines it as void, and the only caller
(lib/pbio/sys/main.c) ignores the result. src/main.c does not include
pbio/main.h, so the compiler never compares the two. This is still true on
master (8aa9986).

With the ARM calling convention this happens to work, but it is undefined
behaviour. On targets that check function signatures it fails: an emscripten
build links with

wasm-ld: warning: function signature mismatch: pbio_main_start_application_resources
>>> defined as () -> i32 in lib/pbio/sys/main.o
>>> defined as () -> void in lib/pbio/src/main.o

and the call traps at run time.

The fix makes the declaration void and includes pbio/main.h in
src/main.c, so any future mismatch is a compile error. Without the header
change, the added include produces:

lib/pbio/src/main.c:70:6: error: conflicting types for 'pbio_main_start_application_resources'; have 'void(void)'

Both files compile cleanly with the fix (virtualhub flags, -Wall -Werror).

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