examples/a440.c: define M_PIf locally for portable compilation

[zordak0x70]

As discussed in this issue,

When building examples/a440.c without _GNU_SOURCE or _COSMO_SOURCE, M_PIf is not defined because it is guarded by those feature-test macros in the libc/math.h header. This causes a compilation error:

./a440.c: In function 'main':
./a440.c:58:26: error: 'M_PIf' undeclared (first use in this function); did you mean 'M_PI'?
   58 |       float s = sinf(2 * M_PIf * WAVE_INTERVAL * t);
      |                          ^~~~~
      |                          M_PI
./a440.c:58:26: note: each undeclared identifier is reported only once for each function it appears in

As discussed in #1495, removing M_PIf entirely from the codebase would be intrusive (it is also used elsewhere, and removing it would create inconsistency with other (x)f macros). The cleanest, most beginner-friendly fix is to define M_PIf locally inside examples/a440.c before it is used.

This PR adds this to examples/a440.c:

#ifndef M_PIf
#define M_PIf 3.14159265358979323846f
#endif

No other files are changed.

Tested with:
~/Projects/cosmocc/bin/x86_64-unknown-cosmo-cc examples/a440.c
No compilation errors. The binary runs.