build: Allow export of environ symbols

This export was introduced in bitcoin#17270 which added
```
//! Necessary on some platforms
extern char** environ;
```

contrib/devtools/symbol-check.py

@@ -47,7 +47,8 @@
 
 # Ignore symbols that are exported as part of every executable
 IGNORE_EXPORTS = {
-'_edata', '_end', '__end__', '_init', '__bss_start', '__bss_start__', '_bss_end__', '__bss_end__', '_fini', '_IO_stdin_used', 'stdin', 'stdout', 'stderr'
+'_edata', '_end', '__end__', '_init', '__bss_start', '__bss_start__', '_bss_end__', '__bss_end__', '_fini', '_IO_stdin_used', 'stdin', 'stdout', 'stderr',
+'environ', '_environ', '__environ',
 }
 READELF_CMD = os.getenv('READELF', '/usr/bin/readelf')
 CPPFILT_CMD = os.getenv('CPPFILT', '/usr/bin/c++filt')