Prevent platform profile error on unsupported systems

If a system does not support setting the platform profile (i.e., does
not have the file /sys/firmware/acpi/platform_profile), then everything
that interacts with it is skipped to prevent errors. This situation is
more common than I expected.[1]

[1] https://github.com/FeralInteractive/gamemode/issues/524

common/common-profile.h

@@ -32,12 +32,18 @@ POSSIBILITY OF SUCH DAMAGE.
 #pragma once
 
 #include <linux/limits.h>
+#include <unistd.h>
 
 /**
  * Path for platform profile
  */
 extern const char *profile_path;
 
+/**
+ * Check if platform profile file exists
+ */
+int profile_exists(void);
+
 /**
  * Get the current platform profile state
  */