build: remove symbol visibility from capture static library

The visibility of symbols was getting extracted from the .a into the
libraries that link_whole the archive. We can simply disable the visibility
from the .a with a custom config.h.meson which avoids it for the capture
library. Since we already double-compile those sources for the shared
library, the shared library visibility is not affected.

This was tested by compiling a simple library which calls
sysprof_clock_init() and ensuring that no extra symbols were found with
`nm libfoo.so | grep ' T '`.

Fixes #60

meson.build

@@ -201,6 +201,7 @@ subdir('help')
 subdir('po')
 
 configure_file(
+          input: 'config.h.meson',
          output: 'config.h',
   configuration: config_h
 )