strbuf: use GREEDY_REALLOC to grow the buffer

strbuf: use GREEDY_REALLOC to grow the buffer

This allows us to reserve a bunch of capacity ahead of time,
improving the performance of hwdb significantly thanks to not
having to reallocate so many times.

Before:
```
$ sudo time valgrind --leak-check=full ./systemd-hwdb update
==113297== Memcheck, a memory error detector
==113297== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==113297== Using Valgrind-3.23.0 and LibVEX; rerun with -h for copyright info
==113297== Command: ./systemd-hwdb update
==113297==
==113297==
==113297== HEAP SUMMARY:
==113297==     in use at exit: 0 bytes in 0 blocks
==113297==   total heap usage: 1,412,640 allocs, 1,412,640 frees, 117,920,009,195 bytes allocated
==113297==
==113297== All heap blocks were freed -- no leaks are possible
==113297==
==113297== For lists of detected and suppressed errors, rerun with: -s
==113297== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
132.44user 21.15system 2:35.61elapsed 98%CPU (0avgtext+0avgdata 228560maxresident)k
0inputs+25296outputs (0major+6886930minor)pagefaults 0swaps
```

After:
```
$ sudo time valgrind --leak-check=full ./systemd-hwdb update
==112572== Memcheck, a memory error detector
==112572== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==112572== Using Valgrind-3.23.0 and LibVEX; rerun with -h for copyright info
==112572== Command: ./systemd-hwdb update
==112572==
==112572==
==112572== HEAP SUMMARY:
==112572==     in use at exit: 0 bytes in 0 blocks
==112572==   total heap usage: 1,320,113 allocs, 1,320,113 frees, 70,614,501 bytes allocated
==112572==
==112572== All heap blocks were freed -- no leaks are possible
==112572==
==112572== For lists of detected and suppressed errors, rerun with: -s
==112572== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
21.94user 0.19system 0:22.23elapsed 99%CPU (0avgtext+0avgdata 229876maxresident)k
0inputs+25264outputs (0major+57275minor)pagefaults 0swaps
```

Co-authored-by: Yu Watanabe <watanabe.yu+github@gmail.com>