Add function prototype for perf_event_open

[andreas-abel]

This pull request adds a missing prototype for perf_event_open.

Without this, the following warning occurs when running make:

rdpmc.c: In function ‘rdpmc_open_attr’:
rdpmc.c:85:12: warning: implicit declaration of function ‘perf_event_open’ [-Wimplicit-function-declaration]
   85 |  ctx->fd = perf_event_open(attr, 0, -1,

[fioushen]

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