Make notificationObserver storage per-file

MemoryBrowserController.m and PokeMemoryController.m both declared
`id notificationObserver;` at file scope without `static`. The two
translation units emit common symbols (nm against the .o files
shows `C _notificationObserver` in each); the linker merges them
despite GCC_NO_COMMON_BLOCKS=YES on the target, so the controllers
share one storage slot for their addObserverForName: tokens.

Latent while runModal forbade simultaneous visibility — the slot
always held the token of whatever window was currently closing.
Reachable now that the windows coexist: open Memory Browser, open
Poke Memory (lazy init writes PM's token into the now-MB-occupied
slot), close Memory Browser first. MB's block reads the slot,
finds PM's token, removes PM's observer. The next Poke Memory
close has no observer to fire — unpause never runs and
fuse_emulation_paused leaks, freezing the emulator.

Adding `static` to both declarations gives each translation unit
private storage. nm now reports `d _notificationObserver` in each
.o file.

Author: morozov

fusepb/controllers/MemoryBrowserController.m

@@ -31,7 +31,7 @@
 
 @implementation MemoryBrowserController
 
-id notificationObserver;
+static id notificationObserver;
 
 - (id)init
 {

fusepb/controllers/PokeMemoryController.m

@@ -33,7 +33,7 @@
 
 @implementation PokeMemoryController
 
-id notificationObserver;
+static id notificationObserver;
 
 - (id)init
 {